I seem to remember reading some time ago about the use of oven cleaner for this purpose. Ask Peggie Hall at Peal Products at www.raritaneng.com/Products/Peal_Products/peal_products.html She specialises in marine toilets but is also a mine of useful information and I think it was she who described this use of oven cleaner on a forum somewhere.
You can buy paint remover specifically for removing paint from fibreglass, try your local chandlers or perhaps a car accessory shop, I have used it to remove sponsors names many times from my race boat when I was water ski racing
if it's not too 'solid' with the GRP then a hard rubbing compound will do it plus LOTS of elbow grease then finish off with something less abrasive (like T-cut) and give it a good polish afterwards - it worked on a Fairline we had and the name had been on it 10 years!
I've just done this job. Used a combination of acetone and a stanley knife blade. Problem with the acetone is it softened the signwriter's paint which then stuck to the pristine parts of the transom as it was scraped off. Not too much of a problem as I polished the topsides afterwards and all the little black bits dissapeared.
Diulette gel. Paint stripper for grp and most plastics. Removes 2 pack enamels as well, but leaves the gel coat unmarked. Only thing is that where the name was will still be fresh and unfaded gel coat. Give the hull a good polish with G32 compound and a good wax based polish, it'll look like new
You won't believe this.
Forget chemicals, blades and nasty stuff.
Use a slightly worn scouring pad, the type with a soft thick yellow side and a harder green side.
Use the green side, keeping it wet, and a moderate amount of elbow grease.
It works brilliantly and will remove rust stains too.
Best tip I've ever been given.
Good luck.