Official Merriam-Webster definition of "gatekeeper" is this: "one that tends or guards a gate or a person who controls access". Therefore, a gatekeeper is someone who controls those who may pass beyond the gate. In the case of this thread, that means online therian communities.
That being said, whoever owns/operates said forum is, by general accepted definition, the official "gatekeeper" of that forum. Logic would dictate that to run a forum, someone must be in charge to make it happen. In the case of TG, the "gatekeepers" would be the mod team because we grant access to new members, deal with problem users/breaking of rules issues, when the forum software has trouble, etc. Someone has to do it, someone has to stand watch at the wall. *shrug*
With regards to the underlying theme of gatekeeping however, people have been using the term "gatekeeping" to discern a good, intelligent place vs. a free-for-all, anything goes type place. A place with rules vs. a whatever people claim is true is true.
It's actually an age-old mentality: we have a place and you want to join our place, so you must abide by our rules, and if you don't like them, you can't come in. It's not a new concept, even here online. If you want to participate, you have to play by the rules of where you want to play. If you don't like that, you're always free to make your own place, behind your own gate.
Personally, I see the value in "gatekeeping", not as a negative thing, but rather it helps to ensure where I'm hanging out is governed by intelligent, nice people and serious discussions, and not fakers, idiots and people who have nothing in common with me.
The bottom line is the same as it's always been: if you don't like the rules behind the gate, go build your own wall.