Hm. I have too much time on my hands, so let's do a deep dive, shall we?
Tbh I don't know if you can even say that gay people have gay face. IMO the gay face shit is just that--BS. There are a lot of straight acting gay men who marry women and don't have gay face. They're passing, because they fool everyone else around them. No gay face.
Zip. Nil. Nada.
I also have questions about that. Does gay face only apply to men? Or does it also apply to lesbians? Doesn't gay face seem scarily similar to the way Nazis used to measure people's heads to determine if they're Jews or not? Isn't Eugenics sooo 1939? "Studies" on "gay face" seems to be just another excuse for researchers to ask for more grant$ for their "research".
Getting back to the subject at hand, a bi face would seem counterproductive. Because bis like both men and women, they've got to act straight enough to attract the women that they want, so they (bisexual men, in this example) can't act too stereotypically femmy/gay or it would run the risk of turning off their potential female partners. Gay men LOVE straight acting gays, so they don't really have to be too gay acting or femmy to attract guys. Gay guys prefer straight/acting men. So bisexual men would have to act really straight to pull both men and women, generally speaking. Not all women are fag hags, after all. I'm not using bisexual women in my example because the only people who hate bi women are lesbians. Straight men love it and gay men don't care. Bisexual men have it the worst because straight women think that bi men are gross gays who can't be tamed by the all powerful P.
Now, if gayface is real, and that's a big IF, then wouldn't stand to reason that bis would have somewhat of a gay face, if they are in fact attracted to their own sex? So, in my own hypothesis, I would say that bis would have somewhat of a gay face, but it would be subdued enough to attract the opposite sex without raising suspicion (that's for all you bi-hating straight women out there).
But really, I don't think your personal appearance has anything to do with your sexuality, for reasons I have mentioned above. And when I say personal appearance, I don't mean the way you accessorize, like dying your hair or wearing white after Labor Day. I'm talking about the immutable, unchangeable aspects of your appearance and personality, such as your bone structure, facially and physically, pre-op plastic surgery, the way talk, and the way you act.