Vented gas cap-- ok in garage that is under the house?

The electric heater, refrigerator, and gas appliance "problem" (if there is one) is normally solved by following building codes, which in most places say that any igniter, spark (motor), or pilot light in a garage must be at least 12" off the floor (many places say 18"). This is because almost all fumes (gas, gasoline, paint, etc...) are heavier than air and will always settle at the floor level.