I first ran into this "gas smell" problem when the Army sent me to the desert (Kuwait). Good thing, too. Saved me from calling the "dumb" gas company or an HVAC tech when it happened in my 3-story townhouse in VA.
When the p-trap in the basement floor drain goes dry, it allows sewer gas to pass into the house. Pouring a gallon or more of water down the drain stops the smell (until the next time it dries out).
Why is it that techs (gas company, HVAC company) can come out to investigate the "smell of gas", not find any "leak", and not even suggest pouring water down the floor drain? It happened to a friend of mine in VA and I couldn't believe it!