Depending on $'s available, you can have the bottom professionally sodablasted or you can strip it yourself. Sodablasting uses bicarbonate of soda which is less aggressive than sand. I've heard varying reports about whether it bites into the gelcoat. Probably if you can find a good person to do the blasting you will have minimal pitting of the gelcoat. Stripping the paint yourself can be done with Peel-Away which is a high alkaline paste. Unlike methylene chloride based strippers, it will not attack the gelcoat.
