Hi!.
I just finished the task of removing my old waterline stripe with a commercial paint remover(the milder one used for antique furniture ). It worked quite well and the gelcoat was never affected. Here is the trick I used: Using a small(1") paint brush, I applyied a very thin coast of remover over a small area at the time. As soon as small bubbles started forming on the painted surface, I quickly scrapped the layer of paint with a sharp scraper, with little pressure, and using a wet sponge cleaned immediatly the area. The idea is that the thin layer of remover will spent all its effect on the paint and wont have any power left to affect the gelcoat. You have to be patient and carefull. The paint lifted easyly, like an almost dry ribbon under the scapper. Almost fun.
Good luck.