Gatekeeper is title bestow to a certain member of the Da'at Yichud to safeguard their vaults of advanced knowledge from falling into the wrong hands. Each vault of the Da'at Yichud has one gatekeeper.
During the event of the New Order, Set Roth mentioned to B.J. that the gatekeeper of the vault in which the Nazi looted to gain knowledge to create advanced war machine to win WW2 was killed while defending the vault.
The fate of others Gatekeeper after WW2 are unknown but the Nazi managed to get another vault in Roswell that contained anti-gravity technology from the US government, which can assumed that the Gatekeeper of this vault was either killed, captured or escaped from the Nazi.