Unless you have a reason to think otherwise, it was probably painted with enamel or possibly laquer out of an aerosol can- its the quickest, easiest, & most inexpensive way for someone to get a decent finish when they don't have professional spay equipment. I make signs for a living & I usually use automotive two part urathane for everything. Sometimes I'll use krylon or rustolium or whatever I can find for interior stuff & filling engraved letters on laquered brass. I clean everything up after it fully cures with naptha. I've wiped enamel off of acrylic letters years after making them to repaint them a different color. I haven't found a single thing it harms (other than enamel). Home depot sells naptha by the gallon for about 8 dollars(US) & I know for certain it doesn't hurt gelcoat. If It's been painted with laquer, I doubt that laquer thinner would hurt youre gelcoat either, but I would test a small area to be sure. If it's urethane you're screwed - break out the sandpaper. I imagine anything that will disolve that stuff would probably eat a hole through the side of your boat.