Try using CLR, but don't leave it on the gelcoat for too long as it will dull the surface. Otherwise, cut it back with 1200 grit wet & dry and buff it with compound.
After cleaning and removing the rust ... use Zirlon clear top coat .. it will stop the rust and put the gloss back onto the gelcoat. It is an all green product that will last 3 to 5 yrs.