I don't see why not to use it. I use Mothers to polish our vintage all aluminum teardrop. Put a little on a rag and rub, rag gets black, and always will as long as you continue to rub. Try it. When you get done with the Mothers you'll have to clean it up with another rag as you would with a car wax.
Mothers works good as does Flitz. I do advise you to put blue painters tape around where you are generating that black stuff on the rag that mary and bob talked about. If your fiberglass is well waxed that black stuff will clean right off but if your fiberglass is bare and oxidized at all that blackness will get into the gelcoat and it is difficult to remove.