The Gatekeeper has yet to step up.
The Guardian has broken a law that only the Gatekeeper has the power to enforce.
The Gatekeeper is unnatural.
If the Others wanted to find a way to keep the Grey God from assuming his role as the Gatekeeper between immortal and mortal worlds, they'd likely find her a good target.
He had newly assumed his position after thousands of years without the Gatekeeper existing, since Xander killed the last Grey God.
Maybe you are not old enough to remember, Other, but the Gatekeeper existed before the Originals.