Im not going to lie to you. Thats going to be difficult to remove without damaging the gelcoat. What kind of paint it is, how long its been there, and the condition of the gelcoat under it will all factor in. You alway want to start out using the least aggressive method possible, in this case I would say a strong all purpose cleaner (APC) and a brush to see if any of it will break loose, which I doubt it will. From there I would move up to something like PrepSol or FinalSol..a gentle solvent that has the ability to remove paint. This may work, but if even that dont, you may have to bite the bullet and CAREFULLY go over it with Laqeur Thinner. This will most likely take it off, but your going to leave a huge dull spot in the gelcoat. If your planning on buffing the boat out once the letters are gone, this shouldnt be a big deal. If your not, it will be. I hope this helps.