I think when we writers most commonly use the term we're thinking of editors, because editors are he most famous. We think of the magazine slushpiles and those assigned to read them, whether they be designated first readers or head editors. We think also, of course, of the agents and editors in the novel-publishing world. Gatekeepers are the ones who get to say to you,
or to put it less gracefully,
Here's the thing, though. The editors and agents aren't the only gatekeepers here. Every one of us who participates in this enterprise is a gatekeeper. It's just that the job of gatekeeping without an official title is far more complex, and more likely to go unnoticed.
Say I'm online and I get approached by someone I don't know, asking to connect or even to have a live hangout with me. How do I know that person is for real, and not some sort of spammer/scammer?
Say I'm at a convention and someone wants to come up and talk to me about my writing, or their writing, or writing, or science fiction and fantasy in general. And I have somewhere to go, or I feel uncomfortable, or I've been deluged by fans (not that this happens to me!) and have had enough, etc. etc. I say no or back out of the conversation. There are plenty of perfectly legitimate reasons to do this. Some of them have to do with mental bandwidth and exhaustion rather than anything else.
However, this is also where inclusiveness succeeds or fails.