No? I mean, it seems that you are asking me if there's a litmus test that if someone (honestly) is in support of gay marriage therefore they and their actions cannot reasonably be interpreted providing a gateway towards accepting alt-right ideas?

The Wikipedia page points out "On social issues like attitudes to homosexuality and abortion, the alt-right is divided ... Some on the alt-right consider homosexuality to be immoral and a threat to the survival of the white race ... Others adopt a more tolerant stance and have praised gay white nationalists" - https://en.wikipedia.org/wiki/Alt-right

So, if 0.5% of alt-right, and 2% of alt-lite are in favor of gay marriage (numbers I pulled out of my ass), then clearly it's not conclusive evidence.

However, since I believe David Koch supports gay marriage based on his libertarian beliefs, and he supports efforts that unfairly make it harder for black people to vote, and he support 'stand your ground' which have make it easier for white people to kill black people - both efforts which I believe are generally supported by the alt-right - then I've already given an example of how "supports gay marriage" is not clear evidence of not being a gateway.