The GT-S was introduced in the 3rd Gen Celica.
It's basically a GT but with bits borrowed from the Supra.
- entire rear suspension (IRS)
- wheels (14x6")
- flared wheel arches (front slightly different because 4 cyl Celica had shorter front sheet metal than the 6 cyl Supra)
- front seats
- sports dash
- various electronic do-dads.
More details here (near the end):
http://members.iinet.net.au/~stepho/gtspecs.htm
Except for the flared wheel arches, all of these are easy to swap from a wrecked Supra.
- Stepho