If you sand the epoxy surface with coarse sandpaper, 40 or 60, the deep scratches will give something for the gelcoat
to stick too. Still not ideal. I'd fill the gouge with polyester putty (Marine grade, not automotive Bondo) and gelcoat
over that. All chemical bonds that way. Note the last coat must be a "finishing" type, not "laminating". Laminating never fully cures if exposed to air, this ensures the chemical bond. Or seal the last coat with PVA.