had this problem a while back on a boat with a slightly faded gel coat,we used
"jif" (white oven
cleaner paste type liquid).
problem is it is quite abrasive and left the gel coat less faded,so ended up doing the whole cockpit
and polishing with silicon wax.
you could also try with engine
degreaser or oven
cleaner as not abrasive,but it will leach out the wax in the gelcoat
making it nessacary to polish after with silicon polish to get a uniform shine.
failing that white spirit will work but DONT use acetone..