The front bar is a GT bar, the rear I think is the same the Pony V-6's got and is 18mm....
Will Gt500 fit? Yes. But that rear bar is frankly too much for a GT. Remember a GT500 has a lot of extra weight in the nose that makes for more understeer, and bigger rear tires which also does the same thing. Unless you have a heavy blower and staggered tires the rear bar is, IMHO, too much rear bar.
I use 35/22's on the cars I setup. My rear bar doesn't work on convertibles, so we'd have to look at Eibach. The balance of such a set of bars is very, very good and while 2mm in the rear sounds small it's about 30% stiffer in actuality and that same 22mm rear bar is itself about 60% stiffer than your stock bar. So as you can see, it's not a small change.