Probably a fuel leak somewhere check the gas cap first often the cap will wear out and cause that. Otherwise try this https://www.youtube.com/watch?v=LYzTyTlnsr8 this video is for oil leaks but the UV dye can be used in whatever to help find leaks and I would try and find that leak quickly because a fuel leak could cause a fire. They sell those kits at any auto parts store and online so try that and that should help you figure out where the leak is coming from. 

One big thing to make sure of first however is does it smell like fresh gas or does it smell like burnt gas if its burnt gas and not liquid gas it could be the catalytic converter has gone out or an exhaust leak so check those things and I hope your able to find the source and if you can't find it yourself take it to a mechanic you can trust and have them help you out. 

If it smells like gas you're leaking gas ! Somewhere you're leaking gas, and it's one of the most dangerous issues in any vehicle.

First and only thing to do, get it checked-out and fixed !