Offset is slightly different, but QP wheels do fit. Last winter we got one first GT's just before it snowed and we ran a set of 19" QP wheels with snows on them.
You can put snow tires on the GT wheels, but if you're getting a second set of QP wheels & tires, the offset will be different. We've got a guy working on some spacers for us (I think he is having a run custom made), so if you can't find them elsewhere let me know and I can see what they will run.
I am assuming that the spacers for the Granturismo would be a standard 5-114.3-30mm wheel spacer? I just bought a set of Sport Edition CD rims from TireRack (4 rims, 18x8), and have a set of Michelin Alpin PA2 Winter Tires. I would like to use 2 spacers on the rears, to adjust the rear track to compensate for the loss in width on the rear. Any suggestions on spacers?