Gatekeeper and Codex can be separated into specific "Hybrid" enemies group, as they are affected both by anti-robotic and anti-organic attacks.
85.202.228.73 11:44, March 21, 2016 (UTC)
In my recent experience, having a high defense makes the Gatekeeper an *excellent* decoy, they can eat an inordinate amount of enemy fire. You just need to move them far enough ahead of your squad that the aliens don't target anything else.