If you were to take up the floor in rear of the GT, you'd find that there's a humongous battery stuffed down there. If it weren't for that, you could probably cobble something up to hold a spare tire.
Regardless of the wheel/tire combination you choose, the requirement is the overall outer diameter must remain within very narrow limits or you will be changing the gearing, speedometer, and odometer. Then, the wheel offset and widths come into play to ensure it will clear the brakes, calipers, and wheel housings. The user's manual lists all of the possible tire/wheel sizes recommended to fit the vehicle.