Acetone... put some on a rag....not directly on the gel... it will remove petroleum and rubber type stains easily.... rinse with fresh water... NOTE: you may want to re wax the cleaned area.
I have used acetone for years to remove shoe scuff marks, dock bumper marks, grease stains, etc. from my boat's surfaces.... most of the acetone will evaporate before it does any damage to gel coat.. but a quick rinse afterward get what's left.
FYI.... for organic based stains such as mildew, algae, (i.e. the brown scum) use an acid based cleaner. An inexpensive commercial toilet bowl cleaner, available at Smart & Final for less than $4/qt. works like magic.