Heat gun, maybe...razorblades, no
Unless you really know what you're doing with a heat gun, use a blow dryer. It may take a little longer to soften the adhesive enough to pull the decals off, but it won't melt your gelcoat. It's too easy to scratch the gelcoat with a razorblade...soften the decal with heat, then pull it off, don't try to scrape it off. You'll need patience, 'cuz prob'ly have do a little at a time.To remove the adhesive residue, spray with good ol' WD-40...wait--in fact, have a beer. Then scrub with a plastic scrubber and wash the area well with a strong solution of detergent and water, followed by a very thorough rinse.This method also works to remove striping and vinyl boat names.