Dimewise

Can I run the C5Z 18" rears on the front of my stock C6 GS? I prefer to run a 17" on the rear.

LS1LT1

I believe the OEM/factory 18x10.5 C5 Z06 rear rims will clear the front of a C6 Grand Sport (and C6 Z06) but the offset might be off a little (might stick out?).
OEM/factory 17x9.5 C5 Z06 fronts do (just barely) clear when used on the rear of a C6 Z06 (with stock rotors/calipers of course) so they should also clear on the back of a C6 GS.