I was on a leg of a delivery of a big Beneteau and we had to top up the tanks from gerry cans while underway and bouncing around a bit. Needless to say we got most of the fuel into the tank but enough of it got on my hands and the gelcoat surrounding the fill cap. The owner of the boat brought out lemon scented liquid Joy which we used on ourselves and the boat. Seemed to work pretty well and is cheaper and smells better than On Off. This will probably require considerable amounts of elbow grease though.
The only caveat is that the boat was coming up from the Caribbean Islands after crossing the Atlantic and the gerry cans seemed to be filled with brown diesel - not the red died fuel IIRC.
Is your stain reddish or just brown?