You may want to check exactly where the smell is coming from. If it is this expansion tank, then the lines connected to it may have lose connections, the connectors may be half-broken off the expansion tank or one of the lines, particularly the one going down under the trunk floor may be clogged.
At the end of the one that goes out below the tank floor there should be this rubber conical thing, hanging there not far from the muffler. If this conical thing is missing, the system does not work - some suction is created there to suck out the vapors. You would then need to buy the rubber thing and attach it to the line with a small clamp.
Lots of posts about it.
There should be no gas smell around the car. If it is there, there is a leak that needs to be traced and fixed.