The Gatekeeper is one of the most powerfull enemies you will encounter. In its closed state it will have an enormous amount of armor, making it a really durable enemy, but it will be limited to use only his cannon attack (wich is quite strong). When the Gatekeeper opens his shell he will be able to use psy attacks, the most notable one is an AoE wich deals a medium amount of damage and also raises any corpse in the area as a psy zombie, and if it kills one of your troops or civilians they will be automaticaly raised too.
When his shell is open his armour will be reduces to half of his base amount, so use this at your advantage to shoot with your most powerfull weapon, as he will close it after being damaged. Just like the Sectopod, he can fly trought terrain destroying it.
You can also use a Flashbang grenade to disorient him, preventing any use of psionics and killing any raised zombie.