This large late-game unit is deployed on normal missions as a pod leader beginning as early as August (mid-November on Legend).
The Gatekeeper's smooth spherical shell creates the illusion of a mechanical unit. However, inside the shell resides a mass of flesh that reflects the utmost extremes of biomechanical integration.
The Gatekeeper has high health and a massive amount of armor, and is unable to use cover.
Gatekeepers have an open and a closed form. They can switch between forms as they choose, and will automatically switch to closed form upon taking damage.
In their closed form, they have greater armor and access to their fairly inaccurate but powerful beam cannon.
In its open form, the Gatekeeper is able to use its psionic abilities, but at the cost of its defense and armor.
The Gatekeeper can use its Gateway ability in its open form, which deals moderate damage in a large radius, and raises any human corpses in the area - including ones killed by the Gateway damage - as Psi Zombies, which it can control and use against enemies.
It is also able to use the Consume ability in its open form, which deals huge damage against a single target in melee range, which heals the Gatekeeper for equal the damage dealt and will raise the unit as a Psi Zombie if it is susceptible and is killed by the attack.
- The Gatekeeper occupies a 2x2 tile area and will break down walls and cover in its path, including mission objective targets.
- The Gatekeeper has 3 actions; its Consume can be used consecutively, while opening its shell only consume an action if it closes right after.
- The Gatekeeper, as an organic enemy, is vulnerable to the full effects of Flashbangs and Gas grenades. Additionally, certain mechanical effects will affect it as well: EMP Grenades, Bluescreen Rounds, although it takes no bonus damage from Combat Protocol.
- When a Gas Grenade cloud is on top of the Gatekeeper, it may avoid moving or acting entirely, opening and closing its shell until all actions are consumed.
- Psionic abilities, including Domination, are all effective provided the Psi Operative can overcome its high Will. With an Alien Psi Amp, it is reasonably possible to take permanent control of one for easy terrain demolition and frontline reinforcements. Its high defense makes it a poor decoy on its own. However, the number of Psi Zombies it is able to conjure can make up for this.
- Psi Zombies raised by a Mind Controlled Gatekeeper are friendly and are able to be moved. However, should Mind Control be lost, Psi Zombies raised this way will die and will count against you in the 'Soldiers Killed' statistic at the end of the mission.
- After using its psionic abilities, the Gatekeeper will remain in its open form until it is damaged once (this includes damage from a stock). In this form they have less armor and less defense. It is advisable to use a high damage attack against it first to exploit this.
- It will explode upon death, blasting a 3-tile range for up to 7 damage.
- The Gatekeeper's AI favors targeting Gateway where it can raise the most zombies, regardless of whether itself or allies are in the blast radius. This can also include groups of civilians if there are no nearby corpses. Re-positioning the squad away from humanoid corpses and leading enemies over them can be a useful strategy.
- Armor shredding effects like Acid Bomb while the Gatekeeper is in its open form will be negated when it returns back into its closed form. It may also switch to closed form in reaction to being attacked.
- In the Commander and Legend difficulties, a closed Gatekeeper is as hard to hit as a humanoid unit behind full cover.