I'm painfully aware of the long cure times involved, as I built a bridgedeck for my boat earlier this year, and the cooler temps I was working in meant that my progress was slower than I'd have liked.
As for gelcoat... from what I've read on the West Systems site, traditional gelcoat does adhere to epoxy reasonably well. Main problems with it are prepping the epoxy laminate properly before attempting to gel-coat, since the amine blush needs to be thoroughly removed.
Polyester and vinylester resins have very strong primary bonding characteristics, but their secondary bonding characteristics are pretty weak, especially on older fiberglass boats, where the styrene has had a chance to evaporate off completely. Epoxy is both stronger in tensile strength and has better secondary bonding characteristics than either polyester or vinylester resin. For this reason, it makes far more sense to repair with an epoxy rather than polyester or vinyester.
Fiberglass doesn't adhere to anything... it has no adhesive properties. Epoxy resin, used in laminating fiberglass, will adhere to fiberglass, gelcoat, glass fibers, fingers, toes, shoes...and most anything else it comes into contact with.
One disadvantage to epoxy resins is that they tend to soften under heat more readily than polyester or vinylester resins. That is one reason many epoxy-based fiberglass boats are painted white... and have caveats about painting them dark colors. A dark colored epoxy fiberglass boat may heat up enough for the epoxy to soften and the laminate to deform. If it cools, it will stay in the deformed shape permanently. YMMV.