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mobyduke;
go to the auto supply store or west marine (if you are rich) and buy a quart of 3m adhesive remover. It comes in a red can. It will disolve the adhesive without harming the gelcoat. spread some on the area to be cleaned, let it sit for a few min. then use a stiff nylon bristle brush scrub the deck. the adhesive will come up and you can wipe the deck with a rag to remove.
I would then wash the deck to remove any remaining adhesive remover.