Gelcoat is pigmented resin. Only the thin, white outer covering is gelcoat. (only 1/16" to 1/8" thick) The rest is layup material such as mat, roving, and a clear or amber colored layup resin.
Marine Tex epoxy will work. Comes in an off white. If you want to match the color, I think there are colorants that can be added. You have to mix and match.
