Gel coat is some tough crap to work on. It fades in a similar manner to lacquer paint, but is HARD and thus resists polishing. From my limited experience with gel-coat (a pair of Sea-doos and a 26' boat), I'd recommend something strong like 3M Rubbing Compound. That should be able to cut through some of the oxidation quite well, and then follow it up with their medium cut polish.
Of course all of this using a rotary polisher and a dust mask.