From Destinypedia, the Destiny wiki
The Gatekeepers are a type of Vex Hydras without a shield that are meant to protect the Glass Throne of the Vault of Glass. Killing them will start the process of summoning Atheon. There are three kinds: a Precursor Gatekeeper, a Descendant Gatekeeper, and a Hezen Corrective Gatekeeper, with a unique chassis.
For an in-depth description of the battle against the Gatekeepers and Atheon in the Vault of Glass raid, see Awaken the Glass Throne
In the original version of the Raid, the first Gatekeeper is encountered upon entering the Glass Throne, surrounded by numerous Precursor and Descendant Goblins and Hobgoblins. Getting too close or opening fire will begin the battle. The Gatekeeper will fire with its Aeon Maul, and summon more Goblin and Hobgoblin reinforcements as it takes damage. Once killed, the Sync Plates for the past and future Transfer Gates will become usable. Inside the portals, two more Gatekeepers, of the Descendants and Precursors, will appear accompanied by their fellow programmings. Killing them allows the players to collect the Aegis and progress the battle.
During the fight against Atheon, Time's Conflux, a Descendant or Precursor Gatekeeper will appear in the respective timeline to obstruct the Guardians brought there. Until they are killed, the plates in the Glass Throne will not activate, impeding the return of the timelost players even after the Oracles are killed.
In the reissued version of the raid, the Gatekeepers have the same function in the fight with Atheon itself, but during the previous encounter, only the regular Gatekeeper appears. While the other players are defending the Confluxes in the past and future, the message "A Gatekeeper appears..." will show on the screen, signaling the arrival of one of the Hydras in the Glass Throne. While the Gatekeeper is there, the portals will be deactivated, preventing travel to the past or future. It is imperative to kill the Gatekeeper to allow the encounter to progress. Once the final conflux appears at the Glass Throne, the Gatekeeper will stop appearing.
List of appearances