Answering your example, gatekeeping won't magically save the industry, but it could keep a classic fallout community from becoming another bethesda's fallout community. People can like whatever they want, but have to understand that niche spaces are to discuss niche stuff. Diluting a community, principally when its a series that had extreme changes to its design principles just weakens it.
And yeah series will not change because of a niche community of course, at best some indies or spiritual sequels with a more oldschool design will show up, sometimes even made by members themselves. And if these go well, maybe more people can come to understand good points about classic design and maybe, just maybe, the main series could change or make a spinoff thinking about that kind of public. These things just can't happen if all communities simply get diluted into whatever is popular right now. It's almost like fighting propaganda.
What you describe doesn't sound super like gatekeeping to me? If you start a classic Fallout forum, then insisting posts stay on that topic is really just sticking to the niche discussion at hand.
When I think of gatekeeping, I think of examples like:
- People who emulate games don't count, only people who play on real hardware
- Gen Z/young people shouldn't give their opinions because they weren't there for the context of the game when it came out, they wouldn't understand
- People who watch anime with dubs don't actually count as fans
- Only people who beat [game] on the hardest difficulty/with a popular challenge can
really give their opinion on the gameplay
etc. Those are the types of attitudes that come to mind when someone says there's gatekeeping in a certain community.
Again, it's all context sensitive and should be capable of nuance. If you're on a forum specifically about original hardware, your opinion about something you emulated probably is not within scope. If you're on a Japanese language forum, then watching things with only English dub is probably not going to fit in with the major discussion.
But again I think there are positive ways to introduce things rather than ways to shut down discovery altogether if it's not within a given path (which IMO is often what the word "gatekeeping" implies).
For instance "You should check out the original Fallout games sometimes, they give some context to the Enclave and Brotherhood of Steel in their original entries that explains why many fans object to their overuse in the modern installments of the franchise" vs. "Well, you were raised on that Bethesda slop, so the quality of the old writing is probably lost on you and your opinion doesn't matter as a true fan until you experience all of these games in the way that I find valid."
One is sharing knowledge and encouraging people to become familiar with aspects of a hobby that you love, one is adversarial and asks a person to achieve an arbitrary baseline of performance before being accepted as a member of the group. If it were a professional association or a hobby that could have dangerous consequences (firearms come to mind as an activity where I do want people gatekept and filtered for safety, for instance) it would be more understandable. But if we're here to talk about games, TV shows, etc. I think the purity testing often drives blood pressure up and enjoyment down for parties on both ends of it.