If the hull is painted this isn't really a gelcoat repair, it's a paint
repair. As long as the scratch/gouge doesn't go into the glass and is therefore just cosmetic, it's a fairly easy fix if you have the equipment
. Just tape the area off close to the scratch to prevent coarser grit scratches farther out and fill and fair the scratch with the fairing compound of choice, WEST/407 if it's big, Evercoat or Rage followed by Evercoat if it's small. Once it's fair prep sand a couple of feet out around the repair with 1000 grit wet. If it's a really long scratch and thus a big area you can get 1000 grit DA film discs but you better know what your doing or you'll blow through like this. Better to wet sand by hand 1000 grit and prep thorough. Then mask off and spray 3-4 coats of the right color Awlgrip, each one going further than the last out onto the area of prep. Let it cure and then blend in the whole repair with 2000 grit wet by hand. Polish it out with 3M Imperial on a foam polishing pad followed by 3M Finesse-It II on a fresh clean foam pad. If you do it right it will look brand new, but if you blow through when blending then you have to re-shoot the whole thing even bigger. Be careful, there's a learning
curve. If it's a big scratch, it can help to shoot a few coats of 545 tinted with the topcoat color onto the fairing compound first and sand it out nicely to make sure any coarser grit scratches or pinholes are filled before topcoating. If it's really small you can get an almost perfect first pass with filler by slicking it with a razor blade and then going straight to wet sanding
. Can save a lot of work if you keep it small and skip primer by doing this.