I've worked in the composites business and done a lot with gelcoat. Don't waste your money, it doesn't actually bond to fiberglass or primer when painted. It also doesn't spray out smooth, like paint will. It's going to require a lot of sanding and polishing, too. Gelcoat also doesn't harden on the surface when exposed to air, unless wax it added. Just paint it. Trust me.
A new hull is made with gelcoat sprayed in the mold, and has a chemical bond with the resin behind it. It comes out glossy and smooth because the mold is a glossy class A finish.