Easiest thing will be to get a wiring diagram for the gtp and then you'll probably have to wire a power and ground in to the 85. This is all assuming the 85 doesn't have power seats. If it does, should be a fairly straightforward wiring job.
I agree with clean8485 because I used to own a 1999 GTP (bought it new in 1999 and sold it after 315,000 km in 2017), and I still have my original 1983 GP. There was not much that I would consider swapping. You could mod your G-Body to use the GTP seats (some effort required), or use the console, but the styles are quite different and would require much "massaging" to get the proper look. The GTP has a smooth, rounded look vice the G-Body which is more square.