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.
Gatekeepers are not immune to flashbang grenades, and they will prevent the use of any special abilities, including psionics, as usual. Gatekeepers are basically restricted to their cannon attack while disoriented.
A Gatekeeper is the "end boss" of the Psionic Gate storyline mission, though they can be encountered earlier than that in regular missions.