I don’t think any researchers studying human sexuality expect that sexual orientation is a matter of simple Mendelian inheritance. Sexuality is more complex than having just one gene or even a small group of genes responsible for how a human being experiences sexual attraction.

That’s not at all to say, however, that the experience of sexual attraction does not have a biological basis. The idea that it does is well supported by evidence, though the mechanisms of that basis are not well understood at all.

Certainly, a "gay gene" is not one of those mechanisms. The idea of a gay gene, unfortunately, is often thrown out as a red herring, More often by anti-lgbtq folks, but sometimes by supporters as well.