Try the Clorox bleach cleaner in the spray bottle. It is a combination of cleaners and bleach. It works great on cleaning and bleaching out stains in hard acrylic plastic sinks and perhaps would do the same for Gelcoat. Spray it on and leave it for a while then scrub and rinse and repeat if necessary. If it is a bad stain on a vertical surface, try a pad soaked in cleaner taped to the surface to allow the bleach cleaner to work. This stuff will make ugly, dirty, badly stained acrylic sinks look like new and does not cause abrasion damage.