Sticker-Off! by Hardline is a must have product for anyone who wants a hassle free way to remove stickers or decals from your wakeboard, boat, water skis, or more.
I think that is an advert. But I have seen this at Wally World and Lowes. Cannot say how good it works.
Use a plastic scraper instead of a razor (it will not penetrate the gel coat use the razor only to lightly lift the edges). 3M also makes a sticker eraser that is available at auto parts stores.
Regardless of what you use.... lot's of elbow grease and lot's of goo gone and TON's of patience!