The Gravity Bomb is an ovoid shaped object covered in pins. When activated it causes the sorounding area to become zero gravity. The gravity bomb itself is weighted to stay grounded, even in zero g.
The Gravity Bomb is used once in the movie when the Zirkonians, Tazer and Razor throw it down the attic stairs at Jake, Art and Lee. The Zirkonians are able to avoid the effects of the gravity bomb by using Gravity Boots. Tom and Hannah are also later caught by the Gravity Bomb but they manage to escape when Tom with the help of Jake destroys it. It is also used in a deleted scene with Doug were he is thrown into the air and lands in the lake.