I had the same problem, the gelcoat gets oxidized from the sun an weather. I used my orbital buffer with a liquid polishing compound, then a sealer finished it off with Rejex. Shines like new now.
This took off the oxidation.
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
This made it shine better than any wax I have used:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1
This is the buffer I used:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1