This nice and simple explanation of how gravity filling works is one of the many useful pieces of information on the APACKS website.
(click image for full story online)
This shows clearly how the fill volume is controlled by the very simple tactic of stopping the escape of air from above the filling liquid. The gravity in the name of the filler refers to the force that causes the liquid to flow, namely gravity rather than piston, auger or positive displacement.
Have a look at the site and see the different types of filling equipment available.