That's because one’s a structural resin that sticks excellently to itself, but doesn’t function well as an adhesive with other materials. The other is an outstanding, nearly universal adhesive that can also be formulated into a structural resin or a coating. 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.