Gatekeeping is a generally derogatory term for when some know-it-all declares, usually without considering the full complexity of reality, a rule by which something is to be included or excluded as a valid example of a set.
For example, someone might say something like the following:
"Real women wear high heels." (implying that women who don't, are not real women).
"It's not a trail backpack if it doesn't have a hip belt."
"It's not anxiety if you can still breathe."
Of course, all of the above examples are nonsense. As can be gatekeeping itself, when it's done without good reason. When done in jest, it can be pretty funny.
There are, however, times when gatekeeping is done legitimately, such as when deciding appropriately who should be granted access to systems or to positions of authority, leadership, or responsibility.
And at this point, we are close to entering the realm of an article on gatekeeping gatekeeping gatekeeping.