And he had to have some Gryffindor traits, to go hunting all those Dark creatures in the first place,
He didn't hunt the dark creatures though- only the wizards or witches who defeated them I don't see him as brave at all so whilst Gryffindors may be all those things you say (I disagree but that's another discussion for another board) their primary characteristic is bravery. Lockhart just isn't brave.
Given that we think Molly wouldn't swoon over him if he were a Slytherin - if she wasn't at school with him would she necessarily know his house?
I actually see Lockhart as being pretty insignificant at school. The least becoming of his features (a weak chin) was probably more apparant when he was a boy so his good looks would have developed more as he grew up. Much like some men who are nondescript at 18 but grow into their looks at 30 (ish) (I'm now trying to think of recogniseable examples but the only ones are English footballers hmmm!) - Maybe Hugh Laurie (House) who was really geeky looking when younger but is now pretty swoonable whereas someone like Jude Law who was lovely looking is going down the pan a bit as he gets older.