I suspect they might have used 3m 5200, which is 'permanent', though I have successfully removed it with patience. Use heat, a razor blade and a 2" putty knife.
score underneath with the razor, then heat, then work the putty knife in and around. you're going to make fractions of a mm progress at a time. It will take patience and time. Muscle it too hard or too fast and you might remove gelcoat.