actual evidence for anything being a "gateway"?
Just because there are many dubious and/or overblown "gateway" claims, typically being somewhat doom-laden, does not mean there are no "gateway" like scenarios which actually occur.
Notably, I might recast a common, and largely uncontentious narrative of the form "inspirational teacher in year X led me to do Y" as a "gateway experience in year X led me to do Y" - the actual events haven't altered, just the language used.
Further, there could easily be competing analyses of a situation, with A claiming that there is no significant gateway effect since very few in its presence went further, while B might be able to point out that the posited "gateway" was crucial in the progression of those susceptible individuals.
It seems to me that the existence of a "gateway" does not demand that everyone walks on through it; and evidence for or against a claimed gateway effect needs to look at the specific situation. But I don't think that it's an unreasonable thing to hypothesize that one /might/ occur in a novel situation, as is done here.