For the wax ring, replace the seal. The flange should be set so that it's about 1/4" above the finished height of the floor. Flange extenders, double wax rings, etc, are options to correct a flange set too low. The toilet should be caulked around the base when set. Did this gas smell problem start after a recent toilet plunging?

If it's the vent that's clogged, you can snake it out.

THere's a few plumbers here to give some more insight.

Hey Speedball, the "Scales above" have disappeared from your posts., FYI

I have a very similar problem that I think started just after a toilet plunging. Does that mean anything significant? I find that when I leave the bathroom with the door closed for an extended period, the smell build up and gets worse and worse. I have replaced the wax ring, but have not yet checked the vent. Thanks for your help.

Although I've never experienced it I guess it's possible to "blow" a wax seal by plunging if the conditions are right. Have you sniffed around the other drains to make sure it's not rotting hair you smell? Good luck, Tom