You could try using a plastic putty knife and carefully peel it off. They can be found in Lowe's and, I imagine, Home Depot. Heat guns must be used with caution. If you have gelcoat sides you could cause delamination if heated too much.
You are probably better off using a hair dryer than a heat gun - less chance of melting something! But I would think that mineral spirits would loosen butyl and it won't harm the fiberglass (but use sparingly around any decals).
More powerful solvents like MEK, lacquer thinner and acetone will remove butyl easily, but it will also take the shine off the fiberglass (or worse). If it turns out you need something that strong, be very, very careful that it does not get anywhere that won't be recovered later.
Acetone and M.E.K. are very strong solvents and not recommended. Better alternatives would be the slower evaporating Xylol (Toluol) or Xylene (Toluene) which will soften the butyl rubber and allow for easy removal. Even so, use care around fiberglas or any type of plastic. A heated (not hot) metal putty knife will help with seams. I use a veneer knife for undercutting the bead or mass of caulk; then follow up with a rag moistened with either of Xylene or Xylol to remove the remainder. As you can see, the blade on the knife lies perfectly flatand the teeth are not offset, so you can safely use it against almost any flat surface.