The Gatekeeper is basically an alien flying fortress, capable of taking immense amount of damage, while at the same time able to unleash powerful attacks on its own. The enemy has two forms - the closed one, gaining a huge bonus to armor, but preventing it from using most of its abilities, and the open one, more vulnerable to attacks but capable of dealing high damage.
Aside from the basic attack, which deals huge damage (up to 12 points), the enemy can use one of the two abilities:
- Consume. The enemy attacks from a close range, dealing damage, healing by a portion of the damage and summoning a Psi Zombie if the attack killed the target.
- Gateway. The enemy attack everyone in a large area, dealing damage and resurrecting all the corpses as Psi Zombies.
Fighting the enemy may become quite problematic if it's able to use the Gateway ability on an area with multiple corpses - this way the enemy can literally flood the battlefield with living dead. Each attack that requires the enemy to open its shell will leave him vulnerable for a single attack (after which it will close the shell) - use that to your advantage by sending the most powerful 1-hitter of your team (Ranger, Sharpshooter) to deal as much damage as possible.
Additionally, you can use the Mind Control ability of the Psi Operative to take control over the enemy. It's not an easy task, as the enemy has high WILL attribute, but it will allow you to acquire a flying, impenetrable fortress till the end of the mission.