smell of gas

Are you sure it's gasoline and not r134a refrigerant? The AC evap coil may be leaking and since it is in the same box as the heater core, you may be smelling it then. (Running the defroster might make it more pronounced, you may also see some oily residue on the inside of the windshield above the defroster vents.) If it IS gas, then you are pulling the fumes in from under the hood most likely, which could be from a leaking injector, fuel regulator, or fuel rail. If not from those then you'll have to trace the whole fuel line to look for any sort of leak. Wouldn't hurt to also check the PCV and EGRs as they can smell like combustion fumes if leaking, although you would also see performance issues.