The Gatekeeper is one of the most powerful enemies you will encounter. In its closed state it will have an enormous amount of armor, making it a very durable enemy. The Gatekeeper opens its shell to use psionic attacks, the most notable one is an AoE which deals a medium amount of damage and also raises any corpse in the area as a psi zombie, and if it kills any of your troops or civilians they will be automatically raised too.
When its shell is open its armour will be reduces to half of its base amount. Like the Sectopod, Gatekeepers demolish terrain by moving through it.
A Gatekeeper is the "end boss" of the Psionic Gate storyline mission, though they can be encountered earlier than that in regular missions if the aliens are advanced enough.
Gatekeepers are a mixture of robotic and organic. They count as robotic for the purposes of Bluescreen Rounds, combat protocol, etc, and are immune to gas and fire, but they are unaffected by Haywire Protocol. Meanwhile they are valid targets for mind-affecting psionics (though they have high psionic defence), and, notably, flashbang grenades work and are highly effective at disrupting their psionics - a disoriented Gatekeeper still has access to its cannon though, which a flanking attack from can still be very dangerous.