I have a GT its on 16" wheels and the ride is really smooth .The car was designed to run on 16" wheels,I wanted 18" but ended up with 16" boy am i glad would not change them now for all the tea in china .If you want you Gt to ride nice put 16" on it end of lol
My GT is on 17s and I found the ride too crashy on broken tarmac (delightful on the good stuff, when you can find any) so I bought a set of 16s and they made a big difference. I didn't like the looks, though. So I upgraded to Koni shocks (kept the standard springs), went back to the 17s and sold the 16s. The ride now is great, still firm and well controlled but much less crashy. I also replaced the Bridgestone/Conti tyres when their time was up with Falkens, which have softer sidewalls and therefore help to absorb the bumps. That made a noticeable difference too. The net is, the GT ride depends on the spec, and it can be tuned after you buy the car, at a cost, naturally. Best to avoid 18 and 19 inch wheels if you think this will be a priority for you. It is for me.