If it's truly gelcoat, do 800 then 1000 then finess-it, then wax. My boat has a pretty thick gelcoat, so the 800 was ok. Wet sand with plenty of water. I probably should have sanded the hull a little more, there were some old decals that had been on the boat and the gelcoat was not faded under them and you can still see a slight difference if you know where to look, between the faded and unfaded.
But I had to stick to 1000 on the painted stripe, it sanded away very quickly down to the gelcoat, I actually had to touch up the brown paint where I started with the 800 grit. google "restoring fiberglass gelcoat" and you will find a bunch of info and videos.
I did a CP23 by hand, and my arms were sore for days afterward. For the CP27 I bought a harbor freight air powered sander to wet sand with. Didn't want to shock myself with the electric sander. Start to finish took me two 4-hour days to sand and wax.