i have the same problem, and its been there for about a year.
but, not from gas. I had a can of Arizona sweet tea, and poured the last inch or so out the window on a hot day, not really thinking anything of it. a few days later after a wash it was still there. and still is after several scrubs/products. its about the size of a baseball but not as dark as the gas stain. but still very noticable. grrr
..my mom warned me about to much sweet tea is bad for me ...if she saw this post she would say ..SE i told you .......RIP MOM..luvs