Paint Stripper. Do not use acetone on gel goat. Do not use MEK. Both of those solvents eat Gelcoat and FRP resins, even cured.
If it makes you feel any better, marine chandlerys sell paint stripper formulated specifically for removing paint from gelcoat, however, you can use paint strippers sold at auto parts stores, etc. Don't leave it on over night.
Note that you should do smaller segments at a time, just like stripping anything else. Note that the gelcoat might have been sanded to accept the paint by the person's who painted it. Scraping the paint and remover off can also cause scratches. I'd use a pressure washer to remove the lifted paint.
Expect to wet sand, then buff the gelcoat once the paint has been removed.