If you want to bring your gel coat back to brand new then you will need some wet sand paper and an orbital polisher capable of about 1500 rpm and some wool pads for it. Start with 1000 grit wet sand paper to strip the oxidation and then move to 1500 then to 2000. By the time your done with that all oxidation should be gone and the surface of the gel coat should be very smooth. Once your done with that then you go to the orbital polisher with the wool pads. I got a polisher at harbor freight for like $40 bucks and the pads for about $20 if I remember correctly. Go to west marine and buy Buff Magic polishing compound, its expensive but in my opinion nothing compares. You paint that on with a paint brush in small section then polish it with the polisher until you can barely see it and then just wipe it off. Your gel coat will look like a mirror again. Once your done your going to have to put a couple coats of quality wax on to protect everything.