hi hywel from the great north east where abouts are you theres a few from uk on here the search engine here is very good and wheels are well covered
the fronts on the gt are the ones to watch as you can foul the foot well if you go to big
Hywel, Welcome to GT.com from an emmigrant Scot (five generations ago!) on the other side of the world - Baz, we have you surrounded
The Backspace of the wheel is the most important measurement on a GT. That is the distance from the mounting face to the inner edge of the rim and should be about 4" - no more than 4 1/2". This and the wheel width determine the offset for the particular wheel you can use.
An added complication is that the offset ("ET" in millimeters usually) is calculated from the centreline of the wheel, while the backspace is measured from the inside edge of the wheel.
This means that an 8" rim (tyre mounting bead to bead size) with a 4" backspace does NOT have a "zero" offset but has an "ET" of about 12 (mm)
positive due to the distance from the tyre mounting bead to the outside lip edge of the rim having to be included in the calculations. So both "ET" and rim width affect what can be fitted under uncut GT wheel arches.
Tyre diameter needs to be kept close to the diameter of the original measurement or else the speedo reading with be incorrect unless recalibrated.
BIG wheels on a GT have a marked effect on ride quality due to their greater unsprung weight - they may look "gnarlly" but make driving the car a chore!
5 1/2" X 15" rims with a backspace ET of 20mm mounting 195 55 X 15 Pirrelli P6000's are my choice!
You're new, so you get some slack, but the FIRST thing to learn here is to do a search before you ask a question. The SECOND thing is to post in a Forum that fits the question. It is described in the Forum heading that the GT Forum (and the other specific model Forums) are reserved for questions relating specifically to that model. Most questions should be posted in the appropriate Technical Forum, such as "3D - Wheels and Tires" which is where I moved this.
Have a look around this Forum; LOTS of answers for you here. HTH