Rust on gelcoat...........HELP!!!

Hello all! I have a quick question for you. I need some help with my gelcoat. I had left a pee-can (...not the type that goes into a pie, either!) in the head for when the holding tank is full. It was on the deck and now it has rusted a ring into it. Not a very severe one, but enough to make you say "Oh, S**T!". How do I get the ring out?
Any help would be much appreciated!
Take care & be safe!