The flares, air dam and door sills on a normal GT5 were riveted to the body, with the seam being finished with an unpainted flexible sealant. As I understand it, the factory's reasoning was that painting over the seams would result in the paint cracking due to differential expansion and contraction rates of the metal and fiberglass.
When I had my car restored, I gave strict instructions to the bodyshop on this which they then ignored without telling me. When they re-attached the fenders they used an aggressive glue and painted over the seam. That was in 2006 and the car still has no cracks in the paint, so it turns out that maintaining an unpainted seam is not necessary if one uses modern adhesives to glue the fibreglass pieces on.
I seem to recall one or two owners with GT5 Panteras reporting that their seams had come smoothed and painted over from the factory (Julian's #9178 comes to mind). Panteras in the 1980s were all built to order and sometimes customized to whatever non-standard specifications the purchaser was willing to pay for (hence the occasional blue velvet interior), so this is an entirely plausible scenario.
Nonetheless, if building a GT5 to strict factory original appearance then, as far as I am aware, for most cars the seams should be sealed and not painted.