Gel coat is usually applied over a fiberglass and is not structural, only cosmetic. In this case, it seems like the gel coat might be mixed in to the whole extrusion..? I don’t know... In any case, it’s preferred to bond epoxy to polyester or epoxy, not gel coat.It's gelcoat as stated... ie polyester resin I suppose; standard stuff.
Curious why you'd want to fiberglass over these joints?
To further spitball the conversation on XPS, I’m curious about adhering these extrusions to XPS panels and then, more or less, glassing over the whole structure. This way you wouldn’t have to trim the XPS at all.