Years ago I created rules for key powers for Gatekeeper since I really missed them from the game. I had posted these ages ago on the old forum, and had tweaked the rules due to the request of some of my local group, but I never posted the revised edition of them.
You should find a zip file attached to this post, which contains the key powers in a style similar to Harbingers character cards. There's also an instruction sheet that explains how to use the rules.