You should not smell gas around your car at any time, running or not. The fuel storage and delivery system of modern day cars is completely sealed including the gas cap. It is very likely you have a leak in the fuel system, and this should be repaired immediately not only because it is costing you money in lost fuel, but also for safety reasons - gasoline is highly flammable and explosive.
I am omitting older carbureted cars here since fuel can often be smelled from carburetor flooding. In any case you should check for leaks.
Ray
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