Source - Halo 2 cutscene in the end of the level Regret.
The Gravity Beam is a newly introduced weapon that covenant ships use(I'm calling it a gravity beam for now). While a Gravity Lift is usually used for ferrying troops and supplies, it can also makeshift into the interesting grav beam. You will first site this ship board weapon in the cutscene as Master Chief escapes a covenant ship opening fire on the temple with it's grav beam on the end of the level Regret. (The covenant ship is possibly either an Covenant Assault Carrier or a CCS-class battlecruiser). To prove that the Gravity Lift plays a role in transforming itself into the gravity beam as a weapon, look at the circumfrerence, which is the outer wall of the beam, and you'll see those same oscillating purple lines on the wall of the beam. As a matter of fact the outer wall ,the circumference of the grav beam, has the same purple color of the ordinary gravity lift. It is very possible that the gravity beam is merely a gravity lift put into maximum power on reverse as most people relize that the as the grav beam hits the floor of the temple the debri appears to be pushed downwards as the grav beam hits it where as a gravity lift pulls upwards. People who believe that this weapon is an energy projector need to realize that Master Chief would be surrounded by steam and would be vaporized by heat as he fell into the surrounding lake. That bright circle was the gravity beam directly over him. The beam also emitted and formed itself on the belly of the ship exactly where the gravity lift would be ---halo 3