Would 'supports gay marriage" be evidence? That seems far from 'alt-right' to me... is that fair?
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.
First, 'libertarian' and 'right' are pretty compatible. See, for example, David Koch. At https://en.wikipedia.org/wiki/David_Koch you'll see he also "considers himself a social liberal,[20] supporting women's right to choose,[21] gay rights, same-sex marriage and stem-cell research.[2][22] He opposes the war on drugs."
He's also a global warming denialist and major funder of many conservative and Republican causes and candidates.
Second, libertarianism is one gateway for the alt-right. I'll quote from the Wikipedia entry for alt-right, at https://en.wikipedia.org/wiki/Alt-right :
> There were also links between the American libertarian movement and the alt-right, despite libertarianism's general repudiation of identity politics.[55] Many senior alt-rightists previously considered themselves libertarians,[56] and the libertarian theorist Murray Rothbard has been cited as a particular link between the two movements due to his staunch anti-egalitarianism and support for ideas about differing IQ levels among racial groups. ...
> After Spencer was fired from The American Conservative, in 2008 he became managing director of Taki Theodoracopulos's right-wing website Taki's Magazine.[73][74] The website initially contained contributions largely from paleoconservatives and libertarians, but under Spencer also gave space to white nationalists like Taylor.[75] In 2009, Spencer used the term "alternative right" in the title of an article by white nationalist Kevin DeAnna;[76][77] by 2010, Spencer had moved fully from paleoconservatism to white nationalism. ...
> It was on websites like 4chan and Reddit that the term gained wider usage, and in 2015 began to grow in popularity.[91] Although there had previously been a strong left-libertarian contingent to these online spaces, there was a gradual rightward turn in chan culture centred on 4chan's politics board, /pol/.
Of course "libertarian" does not mean "alt-right", and "not all libertarians." But the topic here was one of gateway to the alt-right.
Since there is a notable connection between libertarian views and the progression towards alt-right, pointing out that someone has libertarian views is not counter-evidence.