The people whining about gatekeeping are exactly the kind of people you don't want to let in because they probably want to change whatever it is that is the fundamental reason for the group existing and that typically leaves the group without a reason to exist.

Online forums it's rampant. But like, the whole point of any group is to be exclusionary. If you want to make a group for baseball it makes sense to exclude other sports, not because of malicious attitudes but simply because it would then turn into a generic sports group and whatever is the most popular sport would end up getting by far the most attention. Likewise you'd not want to include e-sports into a physical sports group for similar reasons.

Thing is the point of any group is to exclude everyone who isn't there for a particular purpose. And you need gatekeeping in order to do that. Gatekeeping can go bad and be too strict to the point that new members are scared off but that's a different issue entirely.

It's worse in nerd groups because the general dynamics around outcasts lead to loads of social drama and nonsense over petty non-important things. And the barriers to entry aren't so, physical so much as they are skill based in something like a sport. Even in competitive video games there's a skill level involved which is why you see the complaints about gatekeeping from the more pedestrian and lower skilled people.

You don't see as much complaints about gatekeeping from something like say, a mountaineering club because the gatekeeper is the mountain itself. Yeah we can tell you your equipment is shit and we aren't going to let you climb but the mountain doesn't care if you die on it. You can poo poo the suggestion not to attempt to walk up to 4,000 meters in the dead of winter by yourself with not crampons but the reality is the mountain will permanently gatekeep the fuck out of you just as Darwin intended.