Whilst it is possible to say "He is a gay", it's rather derogatory and should be avoided. This is something that just needs to be remembered because using "gay" as an adjective to mean "homosexual" is, in most cases, perfectly acceptable.
OK:
He's gay.
She's gay.
He's homosexual.
He's a homosexual. (Not commonly used)
She's homosexual. (Not commonly used)
She's a lesbian. (Note that the use of the indefinite article here is required)
Not OK (although within the gay community, some of these might be perfectly acceptable):
He's a gay.
He's a queer.
He's a poof.
He's a pansy.
He's a fag.
He's a faggot.
She's a rug-muncher. (Very offensive)
He's a sausage-jockey. (Very offensive)