Removing decal residue from fiberglass

3M adhesive remover when I removed some of my decals worked great. Basically melts the adhesive.

I use Unbelievable! Un-Goo, but it is hard to find at times. Works on anything sticky.

This from 3M.

It was recommended to get the clear bra off my car. I have used it for that, as well as removing adhesive from permits that are on my windshield, as well as on my fiberglass boat and aluminum fishing boat.

Works way better than anything else I have tried (including goo gone and WD40.

I've been reading good reviews for Rapid Remover for Vinyl Decals.

Fiberglass gel coat, correct?
If so, Acetone or any solvent that is "cooler" than acetone....lacquer thinner, 3M adhesive remover, mineral spirits, gasoline, on down to WD40, goof off etc, will remove adhesive residue without harming gel coat. I've used brake or carb cleaner as well on paint, not sure about gel, but would expect no issue with it as well.
Caveat, always try in an inconspicuous spot first.
For mechanical means, an eraser wheel works well too, but gel coat is more susceptible to the heat than painted metal. Go easy with a wheel on fiberglass gel.

Of note, aluminum powder coat is more delicate than paint or gel in my experience. Use more caution on powder coated surfaces.

3m adhesive remover, the best of the three goof off or WD40, have used all three, but the most expensive.

or Goo Gone

WD40. Really!