Dawn dish soap, put it on the gas soaked pants. Then put them in a laundry machine. Works every time.

1. Get pants wet
2. apply dish soap
3 work soap into affected areas by hand
4 while still saturated with dish soap wash according to instructions on the pants.

I have no faith in human perfectability. I think that human exertion will have no appreciable effect upon humanity. Man is now only more active - not more happy - nor more wise, than he was 6000 years ago. Edgar Allan Poe