The Gravity Engine is a device usable as a defensive weapon, possibly in the sense of "a good offense". It appears to generate a purple aura in the visible spectrum while in use, but no other useful details are available.
The owner, and possible creator, of the Gravity Engine is Yeti of Master Payne's Circus of Adventure. A fair surmise might be made that due to Yeti's size and build, and the name of the invention, the wielder of the Engine needs to be quite sturdy him or herself.
In Agatha H and the Clockwork Princess, the gravity engine is replaced by a string of beads which Yeti uses as part of a contemplative exercise involving the gravity equations. Doing so first causes light to bend around him, and finally increases the mass of the horse monster such that it buckles under its own weight.
Possibly relevant outside information
Gravity (or "gravy", since that's what they can turn you into) guns regularly appear in Schlock Mercenary.
The (first person shooter) Half Life 2 has a Gravity Gun.