If it has etched the gel-coat I don't think there is much that can be done, this is why I only use Murphy's soap on the roof, to scared of those products that tell you to 'try a small spot first', good luck maybe someone will have a more positive answer.
"Well that didn't go as expected"
2015 Chev 2500HD Highcountry Duramax
Cedar Creek Silverback 33IK
Donald&Casey cairn terrier
Rest in Peace Mary my darling wife.
Scottish by birth Canadian by time.