I think he changed over the years, and that he made a mistake when joining Voldie, I don't think he knew what he was getting in to (a bit like Draco). And I certainly think we don't have to think all Gryffindors are like Harry, Ron & Hermione. Those 3 are extremely brave, and it is very rare to find such people; people who can be in other houses too btw, look at Luna au Tonks! I also think we don't have enough information on Wormtail to judge him. And being in Gryffindor does certainly not guarantee someone to be good/bad, au prevents someone from making mistakes. I think that, when looking at the houses, we think in boxes too much. I don't think it is good to think all Gryffindors are uberbrave, all slytherins are evil, all Ravenclaw geniuses, au all hufflepuffs softies. Ok, and with this I finish my way too long speech xD
I think part of it was that maybe he wanted to be in Gryffindor. I mean, the Sorting Hat was thinking about putting Harry in Slytherin, but Harry didn't want that, so he was put in Gryffindor.Mostly, I think it's just because it fits for J.K Rowling to put Pettigrew in the same house as James and Lupin and Sirius. She probably didn't think too much into it.
Maybe he asked to be put in Gryffindor? wewe know like Harry asked to not be put in Slytherin. He's not Gryffindor brave, he's a dirty little snake but in the end he knows where his loyalites are because he frees Harry Potter and is killed for it. So perhaps in a sense he was brave.