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.
Possibly relevant outside information Edit
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.