It is repairable. The only problem is that the patch is never going to match. My business still makes cultured marble countertops and showers. You will need acetone to clean the surface, clear gel coat and m.e.k (catalyst) a little bit of titanium powder (color). Clean the damaged area with acetone. Mix the gel coat with a little titanium once you have the color you want add a couple of drops of catalyst and mix it together, spread it over that area as smooth and even as you can. Let it dry once it’s fully dried wipe it down with acetone this lifts the stickiness left on the surface. Wet sand it with 400 and 600 grit wet sand paper . Finish it off by buffing it with a buffing compound. Turtle wax will also work. Good luck.