18x9.5 is most likely going to need to be rolled. You're also going to need to pay close attention to the offset and backspacing. Gto's have very narrow rear fender wells so you're rims are going to need to be centered properly especially when you have a wider tire making tolerances tighter.
What size tire are you trying to run on the back? That has a big effect on the fender well lip clearance. Oh rear Spring height is going to effect clearance too.
I will say I have stock height Lovell Springs out back and can fit my 275/40/17 Nitto Drag radials on a set of stock 17"s with no rubbing. Some people that have 20mm drop springs or lower have to roll their fenders for that same tire. Once you go to an aftermarket wheel, that fender well clearance is a crap shoot. There's also the fact that all GTO's have side to side variances in the cradle alignment. If it biased toward one side at all you'll end up with less clearance on that side. So the only way you're going to know if you actually need to roll the fenders is to buy the rims and tires and put them on the car. Just make sure you follow a wheel offset guide for gto's to make sure you get the rim properly centered within factory spec.
2006 GTO M6
Motor - Stock with Vararam
Bunch of suspension
Bunch of brakes
12.81 @ 110.6 2.03 60'