If you do use Easy Off oven cleaner try not to get it directly on the fiberglass as it can damage the gel coat. I have done this several times cleaning off painted letters. I spray the oven cleaner into a can and then use a brush to paint it on over the painted letters. The oven cleaner will soften the paint and you can wipe it clean. Wash the area and polish if needed.
If you spray the whole area with oven cleaner you can end up with a dull finish except where the old painted letters were which will still be glossy. It is inevitable that you will get some of the oven cleaner on the gel coat. Just keep it to a minimum and wipe it off as soon as possible so it doesn’t get a chance to damage the finish.
Eyehunter