We don't know a lot about the founders, particularly Gryffindor and Hufflepuff. The goblin hinted that the sword wasn't rightfully his. That could mean what Bill had said about goblin made artifacts always being in the ownership of goblins, and other beings simply "rent" them--but maybe it's more sinister. Maybe Gryffindor had murdered to get the sword.
I mean, we can't really wonder about what's going to happen in the next book!
But here's the deal: the second to last time we saw the sword, Griphook was making a run for it in the Gringott's vault. The last time we saw it, it was once again coming out of Gryffindor's hat, into Neville's hands.
How did it do that? It had to be Gryffindor's in some very special sense.