I used to own a detail shop and this is what I used to do striping and lettering removal on older fiberglass boats, cars, trucks, vans, etc.... 3M Eraser Wheel and preferably a cordless drill is all you need. Watch this video, ignore the annoying background noise and there is some good information here. The 3M wheel should cost around $30 depending on where you find it. This product is MAGIC and it will save you time and headache. Just don't use it at too high speed and create too much heat with it. It doesn't take that much pressure to work. Remember that over everything if you choose to use one! Heat = Burnt Gel Coat or Paint. Best of luck and please post a follow up if you use it. Follow up with a gel coat cleaner and then a good polish and wax to the area you worked on. OR, the whole boat!
https://www.youtube.com/watch?v=zsVVizoGANk
Cheers,
Chris