Man that bites. I've ran into this in the past and it's a PITA to deal with. The thing of it is that it's not a surface stain. Gasoline is thin enough it permeates the porous surface of gel coat and the orange dye in gasoline does a great job of dying the gel coat.
What I have found that sort of works is to use a like product without dyes to clean it, specifically "white gas" and a lot of elbow grease with a stiff brush followed up with plenty of hot soapy water. To be honest, even that may not remove it all in which case light wet sanding is called for.