That's not really gatekeeping. In order for words to have meaning they have to preclude the set of all things they are
not. There is a cogent argument that support of collective bargaining is compatible with the philosophical school of libertarianism, and that a liberatarian subscribing to that school of thought would consequently support it. It's not necessarily
self-evident, and you can argue against it if you'd like, but then it's simply a disagreement. Gatekeeping implies an unreasonable barrier being imposed on an ingroup or social identity (not typically a political or philosophical school of thought). For example, "you're not really a fan of if you can't name all the songs from their ."
People disagree about the finer points of political ideas all the time. But drawing a line in the sand doesn't immediately signify gatekeeping, it signifies normal debate. This is especially evident if you compare the example I gave to what you're calling gatekeeping: it would be unreasonable to require all fans of a band to have episodic recollection and deep opinions on every song ever recorded by the band. But it would not be unreasonable to assume a libertarian with philosophical fidelity to the school of thought supports collective bargaining. Again, it might not be correct, but it's not ridiculous.
As a meta point, I'm noticing a trend online where people will give a quick driveby dismissal using terms from the popular zeitgeist. Typical, recent examples include "gatekeeping", "whataboutism" and "gaslighting." Sometimes these are appropriate criticisms to levy at someone's argument and they draw a real insight. But they're also frequently abused or pigeonholed into discussion in contrived ways. From my perspective this seems like a more subtle equivalent of collecting logical fallacies to point out in other people's arguments online. It's symptomatic of something larger: that snark is the modern saboteur of reasoned debate.
Very, very rarely is a single sentence critique of another person's point anything more than a middle brow dismissal. It sounds intellectual but there's no deconstruction or antithesis involved. If you wanted to substantially critique the commenter's point, it would probably have been more appropriate to cite "No True Scotsman" instead of "Gatekeeping". Better still would be to skip the trite idiom altogether and explain why you consider it an error with substantive depth. A pithy comment only convinces those who already agree with you. A meaningfully persuasive one convinces the audience, if not the original commenter.