The Gravity Rifle is a cut rifle from Halo SPV3.
Overview & Background
Essentially, it was the Covenant's version of a grenade launcher. It would use the grenades from the player's invetory and each one would give the rifle a different effect. The Frag would impact and explode, the Plasma would stick to enemies, the Cluster would work similar to a Plasma Caster but have instant detonation, and the Gravity would pull enemies along.
Despite the team really wanting to do it, there wasn't a way to make it work right.
- The Gravity Rifle was also cut from Halo Combat Evolved