Shortened to 'AG unit', these are used on shuttles and on Burns Flipper's grav chair. It isn't known how they work, but they allow incredibly heavy loads to be lifted with ease. The more powerful ones, such as those used on shuttles, seem to be quite valuable and hard to come by, as the Flipper is happy to swap one to make an expensive fake ID for April.
Found at the wreck of Shuttle 709.