Ive done a few and refinish cars for a living. If the gel coat is just mildly dull i wetsand them with 2000 grit sandpaper then buff it with a wool buffing pad and a mild rubbing compound. If there are spots that are a little worse you can sand it 1000 grit then go back over it with 2000. Then ill go over it with a polish and foam buffing pad, then wax it. They make products that you can wipe on that will temporarily make it look better but buffing it will really restore the shine. If you have any more questions feel free to send me a pm