I thought I had posted a review but can't locate it so posting again. This product does exactly what it claims and doesn't seem to harm fiberglass at all. Found it on the web, it's made and marketed in Canada but can be shipped to the states no problem. A little pricey but it's the only thing I've found that really removes cured silicone adhesives. You spray or brush it on, then work at the edges of the adhesive with a scraper (a credit card works great). It takes a while and you need to re-apply to keep working it under the silicone, but what it does is break the bond between silicone and other material. It also turns the cured silicone into a non-sticky gel. Doesn't leave an oily residue like WD40.
I removed a big chunk of clear silicone that a P.O. had used to patch a missing chunk of rub rail. Did a great job and no harm to the gel coat.
Yesterday I removed an old CB radio bracket from the ceiling in my boat cabin. A P.O. had attached it with one machine screw and a ton of silicone adhesive. Very sloppy job and that bracket was very solidly attached. Removal took about 2 hours, and before I started I removed the cabin bunk cushions and put down a small tarp to catch the drips. I brushed it on with a small brush, which uses a lot less of the product than spraying. Completely removed bracket and all the residual silicone. You just need to keep the area wet while it does its magic, be patient, and you'll find it works great with very little elbow grease. Highly recommend this stuff. Here's a link: