I have to cast my vote for Lockhart being a Gryffindor. He's arrogant, vainglorious, and a publicity-hound. And he had to have some Gryffindor traits, to go hunting all those Dark creatures in the first place, even if he did wind up taking credit for what other people did. We saw him behaving in a cowardly manner in the end, but let's not forget, not all Gryffindors have limitless courage, and some never overcome their fears. Neville spent most of his school years terrified of Snape. And Peter Pettigrew, anyone? So turning out to be a coward when the chips are down doesn't mean someone can't be a Gryffindor.
Lockhart is like a caricature of all the worst Gryffindor traits