Try these- hopefull the gelcoat was dirty or waxed -
Tap it with a hammer, then try duck tape to pull it off.
Get a piece of hard wood like oak, use it like a chisel and hit it with hammer at an angle close to horizontal.
Get a single edged razor and very carefully try to cut some off, don't try to pry off, it will take off gelcoat too.
If the epoxy is very thin, try some scothbrite with twisting motion, before going to sandpaper. Try taking some sandpaper and use a twist motion under a finger.
It all depends on how thick the residue is. If it is very thing, you can try a heat gun, then duct tape, but be careful.
