The first thing I'd do is check the fuel line where it connects to the fuel petcock. Many people remove the tank when changing the plugs, as it makes it easier to remove the dogbone that connects the engine to the frame, which must be removed to get to the front spark plugs. When tank is removed the technician has to disconnect the fuel line. If that line was not put on all the way, or the clamp was not slid back on, you could be leaking a small amount of gas from that line. Or the line could have been damaged, which could cause the same thing.
Give it a visual inspection to see if the fuel line is installed properly, then take a clean dry cloth or paper towel and wipe the fuel line right where it connects, to see if there is any gas there.
The other thing is there is a tank vent and vent hose under the chrome bezel that goes around the fuel tank cap. If that vent hose was not hooked up you could be smelling the fumes from there. Remove the plastic chrome bezel and check to see if the vent hose is hooked up. The bezel is held on with allen head screws. You should have an allen wrench that fits in your tool kit.
That's my two cents.