i have a 2005 grand prix gt. i've seen gt1 and gt2 after the name. what is the gt1 and gt2 mean?

what is the widest wheel that i can fit on the car?
where is the best place that i can get reflective bumper inserts?

right now my car is at the dealer getting looked at for a grinding noise while the car moves in reverse and in drive when coming out of park. does anyone know what would cause this? the first time i took it back to the dealer where i got it and they said they couldn't reproduce the noise...imagine that. the next dealer i took it to said it was the shifter, so they replaces the shifter.