An epoxy paint should adhere to a polyester gelcoat. For some reason you can apply epoxy to polyester but not polyester to epoxy.
With the proper removal of the amine blush that sometimes occurs on the surface of epoxy, gelcoat can be applied successfully. (see WEST system’s guide to removing amine blush for the application of gelcoat over epoxy)
The bond between gelcoat over cured epoxy isn’t quite as strong as gelcoat (cosmetic polyester resin) over cured structural polyester resin. …and, yes, due to epoxy’s strong adhesive property the inverse does work better.