One thing to notice is that if you put a gallon of air on a scale, (in a wieghtless container, of course) you would measure zero, exactly. That's because the buoyancy caused by the surrounding air exactly counteracts the weight of the air in the container. If you pressurized the air in your gallon container to, say ten times the outside pressure (or 147.0 psi absolute, or 132.3 psi gauge) it would now weigh nine times 0.01, or 0.09 lb.