Being critical is not gatekeeping. Having opinions about what is good and what is bad is not gatekeeping. Shouting down disagreement and telling people to go away is the hallmark of gatekeeping. It is an attempt to keep out certain voices through ostracization.
Gatekeeping is the 'no true fan of Tolkien' rhetoric, where an assertion is made that anyone who doesn't meet certain criteria isn't part of the fandom.
Oh, you haven't read/watched/participated in xyz? Then you must be fake and don't know what you're talking about. You don't agree with me? You are a shill for someone and don't understand the true meaning of whatever.
What is telling is that people often jump to an assumption about someone else's lack of experience without any knowledge of who they are talking to. So you wind up with Twitter exchanges between a 'fan' and the actual author of the work where the fan informs the author that they don't understand the world and probably didn't even read the source material. Um.
If a discussion has strong 'go away' vibes, or 'long-haired freaky people need not apply' implications, than that's an example of gatekeeping.
Tolkien disliked Dune when he read it. He chose to keep that opinion private, as he saw no benefit to publicly asserting that it wasn't to his taste. He didn't publish a review saying that no true fan of science fiction would enjoy this abomination. His discretion was not limited to this one example.