I am assuming a fiberglass swim platform, is that correct? Is this on a non-skid surface, if so what kind of non-skid surface ?
Since the spill occurred last fall and the stain is orange I would suspect oxidized fuel in the gel coat. Given the time, it may not come out easily, if at all.
Some things to try, if you already haven't:
1. An oxalic acid-based rust-stain remover spray. Spray in on a dry surface, and spray again keeping it wet with the stain remover for 5-15 minutes, then rinse well with water. Another oxalic acid option is Davis FSR - smear it on, let it sit, rinse it off.
2. Scrub with Bar Keepers friend worked up as a paste using a fairly stiff brush. Scrub, let it sit, scrub some more, rinse.
If neither of these helps noticeably then the next option would be polishing or wetsanding / polishing. But if this a non-skid surface with any kind of surface depth, this is probably a limited option.