I think trust is important, yes. Self awareness is, too—sometimes it's enough for the gatekeeper to stop and ask themselves if they've checked their biases. Another tool is for the metagatekeeper to periodically review the gatekeeper's decisions; the metagatekeeper can be the next level up of management, or it could be a set of peers who check each other.

However you do it, it's a matter of "who watches the watchers?"