There are two basic body styles, the notchback, and the fastback. Is this what you're asking? Depending upon the model year, the GT spent some time as both a fastback and a notchback. Comparing a GT to a fastback is... confusing.
Notchbacks have an opaque, reasonably steep-angled panel behind the sail panels. Fastbacks have shallower angle, a bit of a curve to it, and a glass panel behind the sails instead of an opaque one. There are other differences (like the shape of the decklid for one), but this is perhaps the easiest one to describe via a message board while waiting for my coffee to hit me.
The banner at the top of this forum shows sketches of a notchback on the left, and a fastback on the right.
Does this help?