There are two gaskets that can cause a gas leakage in a toilet- one is the floor flange to base gasket, and the other is the flush valve to bowl gasket. Some toilets have an additional gasket between the riser and the bowl, but not all. Replace all the gaskets, I've only had mine last 4-5 years before they stop being gas tight.
The tank is always going to stink, it's designed to do that- it's a big tank full of poop. The trick is to keep the stink confined to the tank and/or vent, out of the bathroom.
I like to wait until the winter to maintain my toilet, then the smell isn't so bad when I have to remove it.