The sun kills GelCoat. But only the outside layer. As a fiberglass repair tech for Dixie Boats. I brought many faded boats back from the dead. And the easiest way to do it. 400 grit water paper.
Get some 400 grit water paper, and a soft wooden block. Balsa wood works great. Cut the paper to wrap part way around the wood. Dip it in water and rub. Not too long in one spot. Don't go thru the gelcoat. Its only 1/8 inch or so thick.The Gel Coat will begin to shine. Then buff with a super fine compound, and wax. It should look like new once you get the top layer od oxidation off.
Do NOT use just your hand behind the paper. You will sand finger prints into the glass. Have to use a block to keep it smooth.