wet sand it. use a fine emory cloth with water, rub until you cant feel the scratch, then rubbing compound, then polishing compound, rub down with acetone, then wax. It'll prob look so good you'll want to do the rest of the boat. Really deep scratches wont come out this way, you'd have to re-do the gelcoat and hope it matches exactly.
sorry that happened to your boat. good luck.
Oh yeah, forgot to say...it looks like a total loss, so sad, time for a new boat.