If a girl says she’s gay she can be referring to herself as being a lesbian or possibly bisexual (liking female and male), or pansexual (liking anybody regardless of gender, [male, female, non-binary, etc]) or less likely (and maybe an improper way) but she could be trying to use it as a term to refer to non heterosexual sexualities like asexuality (no s***** attraction) or aromantic (no romantic attraction). There are other sexualities out there but these are the main ones to pay attention to. If she transitioned to a man, then he would be heterosexual, but only if he used to be a lesbian. If he meant he was bi or pan then he is still gay.