The Gravity Rifle is an unused weapon in Halo: Combat Evolved. It was meant to be a Covenant sniper rifle. The weapon is included in the game's map file, but does not have a first person model, projectile, or string. It will create a bright flash when shot at a nearby solid object, similar to the effect of a flamethrower.
In Halo: Combat Evolved, it is only possible to use this weapon through modding. In Halo: Custom Edition, the weapon can be used in certain maps by using the devmode command cheat_all_weapons.
As such, many aspects of the story such as vehicles, gameplay, design, and story elements didn't make it into the final version of Halo: Combat Evolved.