I have been redoing our boat and there is oxidation on a hull that has lots of gloss. The best way I know, and I have done this many times, is to clean the surface with soap and water. Remove stains and growth with oxalic acid or toiletbowl cleaner. Then with an orbital 5 or 6 inch sander polish the hull with a very fine grit sandpaper. If I have a chip or a stress crack, I repair and polish with 320 paper. Then I move to 600 discs for badly oxidized surfaces and finish with 1200-1500 grit discs for a beautiful surface. Using a buffer with a small pad and automotive type polishing compound you'll have an almost factory new finish.
With this method you'll remove almost no gelcoat and if necessary the whole procedure can be done again when needed. Gelcoat is much more foregiving than a two part
paint and it is much easier to repair.