Experiment to try at home:
Take two glasses. Fill one with air, the other with water. Put a straw in each glass. Put both straws in your mouth. Suck. What do you get?
Answer: More air than water, as air is easier to suck than water.
With a gravity feed system both tank contents are drawn into the fuel system by gravity and by differential air pressure. Simple. A low wing system relies on a pump (which you know about already). Run one tank dry on a low wing system with a tank selector on BOTH and.... you get air.
Hope this suffices to answer your query.
Kermie