It is hard to give exact advice of what I would do without actually seeing the boat but Spd is correct that you should sand it down to the original gel if possible. I normally don't like to use any thing more course than 100 grit when sanding on fiberglass but if the paint is that thick, then I will use 80 grit on a da to get the biggest part of the paint off. As soon as just a little of the gel starts showing through I will switch to 100 grit and finish stripping with a block. This will keep from putting too deep of scratches in the gel that might come back later and will also help straighten the fiberglass. After that I will normally go over it again with 150 or 180 grit before I prime. You should prime and re-sand before painting. As far as the primers go, that will be a matter of choice since I think most any medium build, catalized urethane primer will work. The main thing you have to be carefull of is that no "raw fiberglass" is exposed before you prime with any urethane primers. It all must be cover with either putty or a polyester primer or it will print back through the paint later. Also polyester primers and fillers (automotive bondo's) are not recommended below the water line. To be safe and "right" you should use a Vinyl Ester putty for any repairs that will be submerged. Hope this helps but if you would like for me to go into more detail, feel free to give me a call. 512-423-7889