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.