Interestingly, they're the only enemy in the game that's immune to fire without also being immune to Poison. Mind, Poison is less helpful against them than you might hope since their most dangerous action is long-ranged and doesn't use Aim, but it's still interesting, especially considering the reverse state of being immune to Poison but not fire is much more common. It's also good to keep it in mind if you happen to have Venom Rounds or Gas Grenades/Bombs on hand anyway, so you don't draw an incorrect conclusion about your ability to kill them.
Hardened
Does not use Cover, but is never considered to be in the open.
Like the Sectopod, it's not terribly surprising that a giant armored sphere can't hide behind piddling Cover and doesn't much miss it.
It does, admittedly, get slightly stranger when considering their next quality...
Protective Shell
Has an Open and Closed state. When Closed, it can only move and fire its primary weapon. (It cannot enter Overwatch) When Open, it can use Gateway and Consume, while retaining the option of firing its weapon. Its Open state lowers its Armor by 3 points and Defense by 25 points. It may freely switch between these two states at no action point cost and with no cooldown, but it also immediately returns to its Closed state if damaged while in its Open state.
Inexplicably, other parts of the internet claim that Gatekeepers have three action points per turn. I've no idea why, as they quite blatantly do not. This can be seen effortlessly if you ever take control of one, as while the interface doesn't keep track of the difference between 3 action points and numbers above 3 action points, it does clearly display whether a given unit has more than 2 at any given moment. And no, this isn't an interface problem caused by having 3 by default: Sectopods have 3 action points and can be hacked, and the UI correctly shows that they start their turn with more than 2 action points. Nor is it a weird interface glitch specific to the Gatekeeper: if you order a Gatekeeper about yourself, it will never be able to, for example, move twice and then use an ability/fire its weapon. Nor will you ever see an AI Gatekeeper move twice and perform an action, so it's not that AI Gatekeepers behave differently than player ones in this regard.
I'm pretty sure people are just failing to recognize that opening and closing the shell costs no action points, hence why I mention it here.
Anyway, this is one of the Gatekeepers main gimmicks: it's the return of the Cyberdisc! Only making a lot more sense, among other points having actual cause to use its closed-state tool at times, and also just making more visual sense in general.
Like the Cyberdisc, in practice you can basically plan on the idea of it being vulnerable once a turn, every turn, as it really wants to use its open-state-exclusive abilities and it's not that hard for it to arrange to use them. Unlike the Cyberdisc, the vulnerable state is reliably, meaningfully vulnerable: where an unfolded Cyberdisc was high odds of a shot getting bonus damage via a crit, but could absolutely net you no benefit on a successful hit, the Gatekeeper losing a chunk of Armor ensures there's an actual difference between hitting it while Open vs while Closed. Indeed, even AP Rounds cares about the difference outside Rookie difficulty in specific, since 6 or 7 Armor is enough AP Rounds can't completely bypass it, where 3 or 4 is completely overcome by AP Rounds. The closest to an annoyance in this regard is that 3 points is the default damage variance range in XCOM 2, where eg a high-roll Plasma Rifle shot on a Closed Gatekeeper will be just as strong as a low-roll shot on an Open Gatekeeper. This isn't too bad, and notably secondary weapons and War of the Chosen-added weapon categories generally have smaller ranges: a Bullpup shot on a Closed Gatekeeper is always weaker than on an Open Gatekeeper, all else being equal, regardless of damage roll. This latter point is especially interesting, having me wonder if XCOM 3 might continue this trend of recognizing that wide damage ranges can result in RNG 'overruling' other factors the player is supposed to care about, among other examples of how numbers ought to be tuned in a game that went right over the metaphorical head of the prior game.
Anyway, the Gatekeeper's Open/Close mechanics has a few wrinkles worth pointing out.
First of all, if the Gatekeeper is Open and its Armor is lower than the Shred you're applying to it, the extra Shred won't carry over to the Close-provided Armor. However, if you Shred a Gatekeeper down to nothing in its Closed state, it will not get back those 3 points of Armor by Opening and Closing again; the game genuinely remembers the Gatekeeper's Armor surprisingly precisely. This is especially surprising given that Andromedon Shells don't inherit any surviving Armor from their live version...
Also, not only does the Gatekeeper Close up for free when damaged, but it actually Closes for free if it elects to move. Relatedly, while Opening itself up is a free action, it is an explicit action it has to specifically take, not something it does as a side effect of activating one of the relevant abilities. (This contrasts with Cyberdiscs, which automatically went into their open state when opening fire or lobbing a grenade, with the most obvious manifestation being how you could abuse them going into Overwatch for your own benefit)
Thirdly, the Gatekeeper is protected from a few extra effects in its Closed state, such as Flashbangs. (I haven't exhaustively tested this; I keep forgetting to get around to it, and Gatekeepers are rare enough it's both a pain to test and basically never happens organically)
Fourthly, multi-shot attacks are 'interrupted' by the auto-closing effect: a Banish backed by a Superior Expanded Magazine won't get 6 shots on the Gatekeeper in its Open state, it'll get 1 successful hit on the Open state and then all following shots will be dealing with the Closed state. You're better off trying to Shred and then use multi-shot attacks, rather than trying to use multi-shot attacks on it when it's Open.
Interestingly, the config files indicate the Closed state was at some point planned to actually lower the Gatekeeper's sight range. Also interesting is that there's config file lines indicating that entering the Closed state was at some point a 67% chance rolled separately for each point of damage, though I'm fairly confident that's cut behavior. It's possible it's still implemented and just so ludicrously unlikely to matter without deliberately fishing via Stocks that I've never seen it matter, but I'd be pretty surprised, especially given how much of the config files is cut content.
Also interesting is that the config files seem to indicate that the Gatekeeper originally reduced incoming damage by 33% when Closed. Between this and a few things like the evidence that the Archon at some point had Armor and lost a point each time Battle Frenzy triggered, I suspect Armor and Shred as a standardized concept came relatively late in development, rather than being foundational. That would also fit with how overall cautious the game is about passing out Armor, with it taking a while to show up at all and trending toward fairly low values, even up on Legend.
Consume
A melee attack using base Aim that ignores Armor, and restores the Gatekeeper's HP by 150% of the amount of damage done, that damage being 7-10, and
has no cooldown. If the target dies to Consume and its body can be a Psi Zombie, it rises as a Psi Zombie tied to the Gatekeeper. Such a resulting Psi Zombie functions exactly as per Sectoid Psi Zombies, including that they will die if their creator Gatekeeper dies, is Stunned, Disoriented, etc. Can only be used when Open.
This is the capability that most readily lends itself to a new player having a heart-stopping moment where, after ripping off half the Gatekeeper's health at great effort, it promptly kills someone and undoes literally all the damage you did. Surprise, newbie!
Fortunately, Gatekeepers aren't that fast and this isn't a move-and-melee attack, so even if you struggle to kill/disable a Gatekeeper in a single turn it's not actually that hard to avoid Consume being used, overall. Even better, it does not bypass Parry or Untouchable, and it only heals if it actually does damage; as Gatekeepers are, like many other enemies with a melee attack, prone to heavily prioritizing it if they can perform it in a given turn, these can be used as fairly reliable distractions to neuter the Gatekeeper if your squad isn't up for killing it just yet.
It also can miss, surprisingly. I really wish that wasn't so, honestly; I've already noted that I don't like how melee attackers default to having a gratuitous miss chance, ensuring the RNG can always save you from your own errors, but Consume is a particularly swingy example of this issue, where the contrast between a Gatekeeper landing Consume or not landing it is especially stark.
You should still take the threat of Consume very seriously, though. Being careless around a Gatekeeper can be a very sudden reversal if your squad isn't well-tuned for rapidly killing Gatekeepers, and if any member is a lynchpin to your ability to rapidly kill Gatekeepers, and you leave them in reach of a Consume... whoops! Now your squad is in trouble!
Do note that SPARKs don't have to worry about Consume; it doesn't work on robots, consistent with the broader trend of psionic healing effects being modeled as manipulating some 'life force' organic beings have and machines lack. So it's more okay to leave a SPARK fairly close to a Gatekeeper than most of your organic soldiers.
Narratively, it's interesting to me how this is part of a broader trend of XCOM 2 having psionic powers able to manipulate a 'life force' or whatever you'd call it, where psionically powerful beings can prey upon others to fairly directly improve their own health and whatnot, particularly how it connects to the general implication that the psionic dimension is dangerous to our world; it seems to position psionically active entities as predators, which dovetails particularly nicely with how War of the Chosen successfully illustrates that the Ethereals are Not Good. They, too, came from the hell dimension of psionic predators; it's not surprising they're bad news for us, they're just better at hiding it than a more straightforward predator. Which is an actual strategy real predators use, by the way...
Gateway
Strikes an absolutely massive area for 5-7 damage, ignoring Armor, and attempts to raise Psi Zombies from any humanoid body in the area. This includes any dead
produced by Gateway's damage, and this damage ignores Armor. This has a 2 turn cooldown, and additionally if there are multiple AI Gatekeepers only one will use it in any given turn. The resulting Psi Zombies function exactly as per Sectoid Psi Zombies, including that they will die if their creator Gatekeeper dies, is Stunned, Disoriented, etc.
Note that this bypasses Templar Parrying. Surprise!
Also, to be clear, this is a special ability and doesn't involve an accuracy or Will check. If a Gatekeeper lobs Gateway at your soldiers, they're taking damage. Period. So while Gatekeepers don't have the same potential for lethality as Sectopods, they're still overall an enemy you do not want getting turns if at all possible.
The Psi Zombies it can produce are an unpleasant bonus, much more so than with Sectoids, both because Gatekeepers are much harder to kill, and because Gateway makes them incidentally, where a Sectoid making a Psi Zombie is burning its entire turn on doing nothing but raising a Psi Zombie. To be clear, these are the exact same Psi Zombie as Sectoids produce, so I'll not be covering their stat block here; it's back in the Sectoid post if you need a refresher.
Anyway, this equivalency fortunately extends to the Psi Zombies instantly dying if the Gatekeeper is killed, Stunned, Disoriented, Frozen, and so on, so in the unfortunate event that a Gatekeeper raises 20 Psi Zombies you don't necessarily need to be terrified.
What, you think I'm exaggerating? Lost can be raised as Psi Zombies!
Anyway, yeah, focus down the Gatekeeper. If you can't do that and don't have any of the disruptors to kill Psi Zombies en mass, uh, pray the RNG doesn't let too many Zombie strikes land?
Note that Gatekeepers prefer to aim Gateway for maximum Psi Zombie generation, instead of treating it like a super-attack. Mind, they'll usually catch at least some of your troops with it (For one thing, they pay attention to potential Zombies when determining where to fire, so they do try to target live soldiers), and unlike Consume Gateway can't miss, but in spite of its massive area you'll almost never have your entire squad get hit. (Unless you're really fond of clustering your entire squad together, I suppose)
A curious mechanical note about Gateway is that it's an indiscriminate area-of-effect attack, but it very specifically is incapable of harming its user. This is mostly worth keeping in mind if you Dominate a Gatekeeper, but can occasionally be useful to keep in mind when fighting one.
Anyway, the Gatekeeper has a fairly straightforward hierarchy of preferences: if they're in reach to approach and Consume, they'll basically always do that. If nothing valid is in Consume range, they'll fire off Gateway at the nearest mass of potential Psi Zombies, including your soldiers, possibly without bothering to move. If for some reason they can't do that either, only then will they disdain to fire their eye-cannon.
Said eye-cannon, it should be noted, cannot go into Overwatch (I said so in the Protective Shell description, but I suspect a lot of people gloss over those descriptions) and never runs out of ammo. It's also a completely unique set of visual and audio effects, different from the visuals and audio used for alien plasma weaponry or ADVENT magnetic weaponry. I'm curious as to what it's meant to be. I initially assumed it was a psionically-powered attack, but the game doesn't tag it as psionic damage and the yellow visuals are inconsistent with the purple that psionic powers tend to use, so that seems unlikely.

Aesthetically, Gateway is fairly interesting to me, as the overall animation implies that the Gatekeeper is letting out and then shaping psionic energy using its multiple tentacles. (It's also worth noting that I'm pleasantly surprised the tentacles resemble a squid's tentacles, as opposed to the arms of an octopus or squid, with thick pads marking the end) This is surprisingly consistent with how Psi Operatives animate, just with the Gatekeeper not needing a machine to provide the initial energy, and consistent with the broader trend of XCOM 2 treating psionic powers as the process of drawing out and manipulating an exotic material.
It's also nice that it's naturally intuitive as a result that the Gatekeeper can't use it in its Closed state. Can't get the tentacles out, after all.
Narratively, the name being 'Gateway' is interesting, in part because it's a low-key example of XCOM 2 seeming to shift psionic powers to more of a fantasy logic. It could just be a name chosen because it sounds cool/to connect to the 'Gatekeeper' name, but if it's actually meaningful it suggests Gateway is the Gatekeeper opening a portal to let through psionic entities that are puppeting the corpses. (Or to put it in fantasy terms: opening a gateway to a demonic plane so demons can come in and possess the dead) That this does damage to units in the radius is actually also consistent with XCOM 2 tending to treat psionic powers as like a caustic material that can readily cause harm if not handled carefully, which itself is consistent with how Spectres are written as basically demons and the ending implies that Ethereals and the next game's antagonists both come from the same dimension that psionic energy is drawn from, in the sense that metaphysically the psionic plane seems to be hostile to this plane, in addition to the energies being physically 'hostile'.
I certainly hope this is all the new logic, and not just a wild coincidence the devs didn't intend, as it's interesting worldbuilding and has a lot of narrative and gameplay potential as a foundation.
ExplosiveThe Gatekeeper always explodes on death, doing 5 damage with 2 Shred to everything in 3 tiles of it.
Unlike other exploding enemies, the Gatekeeper does have an icon for the fact that it explodes on death, but this isn't the icon in question. I'm using this one for consistency, and because I'm not sure how to get a version of the proper icon that parses correctly, visually.
In any event, Gatekeepers exploding on death is, like with Sectopods, one final reason why it's generally dubious to melee them. Consume is more of a reason, to be honest, but it's still generally better to not hurl your Katana-wielding Ranger at a Gatekeeper if you don't have to, no matter how much of a help it is that they can't miss and are unimpressed with the Gatekeeper's high Armor.
A wrinkle unusual to Gatekeepers to keep aware of -and which is very relevant to the 'meleeing them is a bad idea' point- is that their combination of flight and exploding on death makes them uniquely prone to punching holes in rooftops by virtue of exploding atop one. Quite massive holes, since the explosion is scaled to their size. This is particularly pertinent to VIP Extraction missions, as those have a fondness for placing the evac point on a rooftop, and since Gatekeepers are the other 'boss' enemy of the base game a mission containing one or more Gatekeepers will likely have one of the pods spawn near the evac point. This can occasionally result in an awkward situation where the Gatekeeper activates, charges to nearby the evac point, and now if you kill it the evac point will be destroyed and a new one generated elsewhere. Hope you have a Psi Operative with Stasis, or enough time to account for where the evac point jumps to!
It doesn't crop up as readily as Archons blasting evac zones with Blazing Pinions, but it can be more frustrating since it can happen literally during the pod activation, where not even preventing the enemy from getting a turn helps. Fortunately, Gatekeepers take long enough to enter rotation it's possible to beat the game before this is a concern, particularly in the base game...

Anyway.
The Gatekeeper is, of course, the other boss spawn routine enemy in the base game, and so like Sectopods initially you normally won't encounter them until the tail end of a given mission. Also like Sectopods, if there's multiple Gatekeeper pods in a given mission only one of them will be held to this outside of Chosen Avenger Assaults. And still like Sectopods, a plot mission has a special encounter with a Gatekeeper that's meant to be your first encounter with the enemy type. Unlike with Sectopods, this plot-mandated Gatekeeper encounter places the Gatekeeper at the very end of the mission, and it's quite likely you'll end up fighting Chryssalids at the same time as fighting the Gatekeeper, instead of fighting the Gatekeeper alone, so it can easily be a much rougher fight.
Unlike the Sectopod, as far as I'm aware there's not any evidence there was supposed to be a 'prototype Gatekeeper' or anything like that. That's an interesting difference in development, even if it doesn't matter to the final product, and I'm a bit curious what underlies it.

The Gatekeeper's combination of high Armor and high inherent Defense makes it difficult to quickly kill them if you're not prepared for them. It's generally crucial to have explosives at the ready for them, letting you bypass their Defense while shredding their Armor. Holo Targeting is also a good idea to have handy, since it's one of your only ways of fighting against high inherent Defense. In the base game, Grenadiers are practically mandatory. Either that or Psi Operatives, who can potentially bypass the issues entirely, whether by inflicting severe Armor-and-Defense-ignoring damage or by straight-up Dominating it so your enemies are the ones who have to worry about killing it. In War of the Chosen you're more flexible, thanks to the Training Center overhaul, the addition of Bonds, and the fact that all three new classes bring things to the table that help against Gatekeepers, but you still need to be ready for Gatekeepers in a way that just doesn't apply to most enemies.
Once you've Shredded their Armor, Bluescreen-backed Banish Reapers or Fan Fire Sharpshoooters are some of your best options for killing them quickly and reliably. Even on Legend, a Bluescreen Rounds Fan Fire on a Gatekeeper with no Armor will remove half its HP on just the Bluescreen Rounds' bonus damage; as you have to do damage to inflict Shred, this will probably be a kill-shot outright.
I personally prefer the Sharpshooters for this job, in part because they're sustainably effective against a wide variety of tough targets where Banish is single-use. This gets especially important when multiple Gatekeepers are in a mission: if your sole answer to a Gatekeeper is to break out Banish, you're going to have a problem when a second one hovers around the corner.
Surprisingly, in spite of its psionic power a Gatekeeper is actually perfectly susceptible to being Dominated; their Will is pretty high, but not nearly as high as I would've expected. Indeed, they're actually a fantastic Domination target, which can be particularly nice in the base game's version of the final mission, as they'll soak a tremendous amount of punishment and do decent damage very reliably, with Gateway potentially providing many distractions the AI will foolishly prioritize over the Gatekeeper due to how much easier to hit they are, and Consume allowing even a fairly beat-up Gatekeeper to keep going without expending any of your precious resources. The only significant flaw with Dominating a Gatekeeper is that you won't get their corpse, which you kind of by definition want if you've got access to Domination... but that's not an issue in the final mission.
Unfortunately, in War of the Chosen the final mission is broken up into two parts, and while I consider this overall for the best it does hurt the utility of Dominating a Gatekeeper in the final mission: they'll only show in the first part (Neither 'boss' enemy can appear in the second part), and the game doesn't carry Dominated units into the second part, where you most need the assistance. (Incidentally, this is one of the more subtle ways War of the Chosen made it less appealing to stuff Psi Operatives in their closet and break them out for the final mission: less utility from Domination in the final mission)
So in War of the Chosen, Dominating Gatekeepers is a noticeably more dubious prospect unless you're willing to invest the effort of breaking the Mind Control (Which to be fair can be trivially done with Stasis), or are on a mission you can't loot corpses in anyway. (eg VIP Extraction, Avatar Project Facilities, etc)
Still very effective, mind.
The Gatekeeper Autopsy is necessary to acquire your third-tier Psi Amps, and so is crucial if you want to make your Psi Operatives the best they can be. If you don't care about that, then it's irrelevant, and indeed you should sell off Gatekeeper corpses if you have no intention of using Psi Operatives, and if you are using Psi Operatives you should sell off their corpses once you've upgraded to Alien Psi Amps, as there's no further use for Gatekeeper corpses.
In practice, Gatekeepers take so long to show up, and the Gatekeeper corpse count needed for Alien Psi Amps is so demanding relative to how many corpses you're liable to get, that Alien Psi Amps are, as noted before, something of an unrealistic luxury, unlikely to see use in real play. Whoops.
So there's a decent argument that you should just sell Gatekeeper corpses, period, if you're playing to win instead of wanting to see all the game's content, especially in the base game, and also especially if you don't play on Legend.

Also, the Autopsy graphic confuses me, as it seems to present the Gatekeeper as a robot, rather than a squishy brain sitting inside an armored shell. You can see a couple of dark lines that are hopefully meant to be a couple of tentacles, but that's it. It has me wonder if at some point the Gatekeeper was a robot, especially in conjunction with it being susceptible to Bluescreen Rounds and EMP Grenades given how there's no obvious reason for this in the final product.
Also, just as Sectopods blow up into tiny fragments yet Tygan gets to somehow cut open an intact machine, Gatekeepers explode into tiny fragments but Tygan somehow gets to cut into an intact Gatekeeper corpse. There's also the issue of 'wait, how did your crew haul the body back in the Skyranger?', just like Sectopods...
Ah well. XCOM 2 pretty well game-ified the Autopsy mechanics in the first place. It clearly wasn't a priority to make them realistic and sensical, and I don't necessarily mind that.
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The Gatekeeper's design is possibly my favorite design in XCOM 2. The initial appearance of a giant floating eyeball is striking, makes a reasonable amount of sense for armoring a target relying on some kind of anti-gravity effect, and is just nice all-around. Then the reveal that actually it's some kind of Mother Brain thing is startling, yet vastly more visually intuitive a surprise reveal than the Cyberdisc's (Which always raised questions of how all of it was crammed into that disc form), and as a nice bonus the game is actually being setting-consistent with Enemy Within in a subtle way: remember, the Mechtoid's Autopsy informs us that the heavy armor interferes with the Sectoid's psychic abilities. Then look at how the Gatekeeper has to peel away its armor to access its two psychic abilities. The game doesn't draw your attention to it or explicitly spell it out, but the Gatekeeper is clearly a continuation of the 'heavy armor blocks outgoing psychic powers' concept. That's a nice bit of attention to detail, the kind I rarely see in AAA-type titles. It'd be nice for the setting to address why this is apparently a one-way rule, but whatever, I'm at least glad to see that being continued; far too often 'explanations' in settings are one-off statements that are ignored in all other situations they ought to apply, rather than built on and held to more or less consistently. It gives me some hope that XCOM 3 might bake this notion into the actual mechanics, like discouraging putting your psionic troops into super-heavy armors because it imposes Psi Offense penalties or similar.
Anyway, I'm genuinely curious as to whether Mother Brain is an explicit inspiration for the design or not. It could just be a weird coincidence -It seems most likely for Gatekeepers to have been inspired by Terror From the Deep's Tentaculats, which are also floating brains with one eye and tentacles and the ability to produce zombies, just underwater and without visible armor- but that doesn't mean it isn't also drawing inspiration from Mother Brain. After all, the Andromedon is drawing inspiration from Calcinites and Big Daddies. So it's not like 'is drawing inspiration from Terror From the Deep and elsewhere' is unprecedented.
In any event, it's a cool design.

On a different aesthetic note, something interesting that's easy to overlook is that the Gatekeeper's hovering involves a visible orange/yellow glow apparently supporting it. This is interesting because XCOM 2 is extremely consistent about representing pretty much anything to do with psychic powers with purple effects; War of the Chosen is willing to skirt this rule a little, with the Ethereals punishing the Chosen being primarily a white effect (The edges bleeding away are still purple, though), and the Warlock's arms crackling with red-and-purple at higher tiers of training, and a few other things that cleave to it less absolutely, but the base game is completely consistent about this, and even War of the Chosen's deviations never entirely remove purple from the equation.
I suspect I'm far from the only player to initially assume the Gatekeeper was floating psionically, like Ethereals, but the visual signaling seems to imply the intention is this is not psionic in nature. Which makes sense in conjunction with the Gatekeeper's design cleaving to the 'heavy metal interferes with psionic powers' rule!
It's also interesting to note that it's not the blue glow used by Gremlins and BITs, suggesting it's either a different anti-grav technology, or possibly it's some strange biological capability, something inherent to Gatekeepers that isn't psionic. XCOM 2 is sufficiently willing to credit its aliens with improbable capabilities that are presented as biologically-rooted and not psionically-connected, such as Faceless shapeshifting, that I'm not willing to dismiss this possibility, even though biologically-based anti-grav is one of those things scifi is usually loathe to include. Notably, Spectres in War of the Chosen can fly with no apparent attempt by the game to visually justify this (Or to narratively claim it as psychic power usage), and I've already been over my suspicions that they're intended to be some manner of Void demon, and Gatekeepers are firmly connected to interdimensional travel by the game -I won't be at all surprised if XCOM 3 decides to reveal that Gatekeepers are just extradimensional aliens like the Ethereals who don't play by our rules, just in a somewhat friendlier package than Spectres.
It's a little disappointing we don't really get any kind of explanation of what's up with the Gatekeeper, though. This applies to basically all of the new Alien breeds to some extent, but Gatekeepers stand out because they're powerfully psionic. XCOM 2 doesn't acknowledge the previous game's claims about the Ethereals being on the hunt for 'the Gift', but the new plot is still premised under the idea that the Ethereals need humans in no small part because of their psychic potential. So... how does a being with clear, significant psionic ability fit into this framework? It's not like it's impossible to justify, but it's the kind of oddity that really needs to be addressed, as even if the devs have an explanation in mind, and even if it's a good one... without explicating it, it just looks like an error that undermines the plot's coherency.
So out of all the enemies that go largely unexplained, the Gatekeeper is the most problematic to be left unexplained, at least until War of the Chosen added in ADVENT Priests.
On a different note, that plot mission is... problematic. Well, not the mission itself exactly, but how it connects to the events that follow: you have a Gatekeeper pop in through the Psi Gate, the Gatekeeper is arguably the most dangerous regular enemy in the game and the narrative treats it appropriately, and your crew looks at the Psi Gate and decides to grab it, install it in the Avenger, and turn it on.
What should happen in response to this blatant act of rampant stupidity is that another Gatekeeper -or something worse- pops inside the Avenger and wreaks a lot of havoc, possibly singlehandedly destroying X-COM the organization because seriously you idiots literally installed a back door into your only secure base and then took no security measures whatsoever.
What does happen is that everything is fine and the only casualty is Shen's special snowflake Gremlin gets mildly fried when she tries to send it through the portal.
I can squint and kind of understand how the devs created this insanely stupid sequence, in that the endgame hinges on X-COM using the Psi Gate and bringing the Psi Gate aboard the Avenger causes it to be intuitive for X-COM to study the Psi Gate no matter where the Avenger flies off to, but it's really, really difficult to be generous and overlook how utterly broken this is. I am not exaggerating in the slightest when I say this should, from an in-universe standpoint, have been a lethal mistake.
Part of the problem is that nothing about how the game comports itself acknowledges the danger. Just having a bunch of X-COM soldiers standing in the room, weapons aimed at the Psi Gate when Tygan and Shen first activate it, plus some technobabble from Shen about how the Psi Gate can't possibly be re-opened by the Ethereals while it's on the Avenger because Technobabble Reason, would've gone a long way to make it possible to gloss over it. It would still be an insanely dangerous act, and it would still be bizarre how it utterly fails to have appropriate consequences, but an acknowledgment that logically speaking this is incredibly dangerous would make it a lot easier for me to swallow the plotpoint. As-is, I'm left to wonder if the dev team didn't actually operate under the logic I laid out in the previous paragraph, and would've written this insane nonsense regardless of whether the pressures I'm imagining led to the decision existed.
XCOM 2 is overall better-written than its predecessor, but moments like this left me wondering, in playing through the base game, if it was overly optimistic/generous of me to think XCOM 3 might finally have a good plot. Thankfully, War of the Chosen is a surprisingly big step up in plot quality with no new horrific missteps of this sort (There's missteps, I've covered them already, but not ones like this), so that's promising.