It is VERY difficult to diagnose a problem that doesn't exist when I am on site.
Call a plumber, tell him you have a gas leak. He'll look for a open trap, or a gas line.
Call a heating guy, tell him you have a gas leak. He'll look for a gas line, or a broken flue pipe.
Call a mechanic, tell him you have a gas leak. He'll look at your car's gas tank.
Call a painter, he'll think you've got an open can of paint remover.
See where I'm going with this?
Calling something "gas", can be a number of things, although the companies you called, should have asked you about the smell, and pointed you in the correct trade, if not themselves.
Because the problem wasn't solved, I assumed that the homeowner wasn't availible to describe the smell, since anytime a homeowner is availible, they talk your head off.
There is no one that can master the trade of an entire house. "Handymen" try it, and it doesn't work out to great. Same with home inspectors.