sorry I didn’t read properly. To cover rough patches use glass woven mat/tissue – say 150gsm (lightweight) in epoxy or fibre glass resin. Again you must abrade well as gelcoat contains wax which floats to the surface so that it goes off.
Nothing sticks well to unabraded gelcoat. not paint, epoxy, or glass resin
Woven tissue adds a lot of abrasion resistance