how or why do you think Peter Pettigrew,aka wormtail, became a Gryffindor?

Well, the Sorting Hat took Harry's opinion into account for his Sorting. Maybe the Hat did the same thing with Peter. Maybe he wished to be in the same house as James, Sirius, and Remus. Or maybe Peter at that point in his life, before the influence of of his friends' popularity and before the appeal of the Dark Lord reached his ears, he had potential to be a great Gryffindor. Also, one of the Gryffindor qualities is loyalty, and for most of his years at Hogwarts, he was incredibly loyal to his friends.

That is called Endless Mystery of Hogwarts :))

i think that the sorting hat could sense that someday peter would do something incredibly brave. In the seventh book peter saves harry from the malfoys house, but as soon as he does that his silver hand chokes him to death, so truly he died being Valente and courageous, and to do the right thing.

almost everybody is innocent when is 11 years old & the sorting hat didn't think about it! maybe the sorting hat isn't perfect at all....

The hat sorts too soon, Dumbledore has said that before, hasn't he? Besides plleenty of people change from when they are 11 or so.