Not to come across like I'm denying that gatekeeping is a real nuisance across all kinds of hobbies, but to be totally frank I don't think the things you're describing here are gatekeeping? If people are engaging in a hobby that involves a lot of care for the text, like theorycraft or analysis or like roleplay in your example, then I think they're within their rights to be annoyed when someone else isn't playing in the space with the same level of care. If they were rude, then they were rude, and that sucks and should change, but I feel that the term gatekeeping itself refers to a very distinct phenomenon that is different to what happened there.

This is hardly just limited to "newgens", either... if you asked me who the most annoying group of fans are, who always want to play in the big pool but never want to learn to swim, I would say it's the homestuck oldheads who think they know everything cus they read the comic 10 years ago but don't actually remember anything from that far back. If I could click my fingers one day and suddenly stop having to deal with these kinds of people when I'm trying to perform earnest literary analysis of a comic that I love, well, I think those two fingers on my one hand would at the very least touch...... does that make me a gatekeeper. i donno. call me Cerberus i guess 😔