Start with hot soap and water and wash it out good, rinse it a million times, wash it again, then try the other suggestions. you need to get all the gas residue out or it will never stop smelling.
Well, here's the results. First, it's probably cheaper to buy a new gas can. Having said that, here's what worked reasonably well for me.
Michael nailed the key point - there was still residue on the (inside) bottom of the can. I poured about a quart of lacquer thinner in the can with a handful of 1/4-20 nuts and shook the crap out of it. I let the thinner set overnight, then poured it out and rinsed with water and dish soap 4 or 5 times (I lost track). It might have been a good idea to leave the soapy water in the can overnight, but I was in a hurry and it smelled OK (trace lacquer thinner, but nothing like before).
My canjo is allowed in the house now - hey, the basement counts :)