Wow. I agree with my colleague--there is no evidence I'm aware of that Gertrude is not Hamlet's mother. Hamlet calls her his mother, King Hamlet calls her his wife, and Gertrude calls Hamlet her son. No one, at any point in the play, ever suggests anything different.
So I ask myself what might have prompted such a question. I can really only think of one thing that might prompt such a thought--Hamlet doesn't treat his mother very well. While that is not the kind of behavior which should be condoned, we understand his anger with her (no need to recount her offenses here). Yet he does, in the end, enlist her help and admits to her his madness is an act--something he told very few people including Ophelia, whom he loves.
I, too, am curious what prompted the question.