If it is a strong smell of gas and you haven't been running the motorcycle, chances are that there may be a leak somewhere, and you just haven't found it yet. It could be slow enough that the gas has time to evaporate mostly before you see it. ALSO, DO CHECK THAT YOUR PETCOCK is working properly and check the oil in the engine crankcase - if the petcock isn't shut off and the float needle doesn't completely seal off the inflow of fuel from the tank, your engine/crankcase could fill up with gasoline, which would be a VERY bad thing if you then went to start it.
The gas cap IS supposed to vent to the atmosphere - if it doesn't, then you get a problem with fuel starvation from a partial vacuum that is created in the fuel tank when the fuel level drops.
But, I don't smell a strong gasoline smell when the cap is screwed in place - in fact, I'd have to work very hard to smell any gasoline smell at all unless some was spilled out or the tank was overfilled. If your gas cap doesn't seem to screw in tightly, maybe it isn't seating properly and allowing fumes to come out but I haven't had this happen to me before.
Lastly, check the underside of your fuel tank. Do you have the fuel overflow hose connected? It should go from a small tube on the underside of the fuel tank toward the front (the top of the tube is the small circular hole you'll see on the side of the filler neck when you unscrew the gas cap). Gas overflow from the tank will drip down this tube, and there should be a hose connecting the bottom of the tube and run back along the frame and downward to drip onto the ground, to the side of the rear tire.
Lastly, lastly, check your tank carefully along the seams for signs of dampness or paint bubbles. IF the bottom of the tank is rusting through one will often smell gasoline odor before there is a significant drip-drip-drip, often the paint will keep the gasoline in check until the gasoline eventually dissolves through the paint.
Good luck-
