Polyester resin or epoxy resin works the same way with gel coat. The gel coat only provides the smooth outer surface, has nothing to do with the strength or adhesiveness of the resin. Just in case, you can use kevlar, S-glass, E-glass, carbon fiber...etc, with either resin AND gel coat.
An interesting and important fact about the relationship of epoxies and polyester resins is that epoxy will adhere to cured polyester, but polyester will NOT bond to epoxy. Maybe thats what they meant but West Marine makes gelcoat and right on the can it says "Use with polyester and epoxy resins for reliable, high-strength coverings, laminates, and repairs"