Well, a gatekeeper is the opposite of a barker. A gatekeeper would try to keep people from ever knowing, or even thinking about, about what was going on behind that door.
First of all, a gatekeeper would probably do unobtrusive things to keep passers-by from even noticing that there was a door there at all. He might stand and/or place things in front of it, trying to block it from view. He might put a large mural of a brick wall (or of a dingy alleyway) over the door, to convince people that there is no entrance (or that there is a worthless entrance of no interest to anyone, that not just can but should be disregarded) there.
If you looked his way, he might ask you for the time, or point out an airplane or cloud formation (“Hey, do you think that’s a chemtrail?”) up in the sky, or someone across the street (“Hey, isn’t that Britney Spears over there?”), to distract you and divert your attention from the doorway whose existence the gatekeeper was hoping to conceal.
But if you did happen to notice the door and ask about it, the gatekeeper would probably not try to change the subject, for that could make you curious or even suspicious. Instead, the gatekeeper will typically put on a big friendly smile and offer to take you inside and show you all around the place. If you happen to have one hour available, he will gladly provide you with at least 61 minutes of hallways of doorways to examine to your heart’s content. He will be very cooperative, and exhaustively lead you down every hallway to every door (except the one that matters). And by the time you leave, you will be convinced that you have been shown all the doors, and there is nothing of interest behind any of the doors in that establishment — you might even later, probably unwittingly, assist the helpful cooperative friendly informative gatekeeper in concealing the truth about that place.