The Gate Generator, or Demon Gate, was used by Zeikfried to attack Filgaia, seemingly by ripping the sky open and striking the land with "Swords of Light". It was first seen attacking Adlehyde on the day of the Ruin Festival.
The Generator also has transport capabilities, transferring Rudy, Jack, and Cecilia from the Demon's Lab to the Generator. Zeikfried attempts to attack the trio using the swords of light, however unsuccessful so.
The Generator creates subspace where the trio remain suspended in limbo. Zeikfried sends Amplifiers to attack them, however is again unsuccessful. He then taunts the trio, saying that battling in subspace is extremely risky and they could've died. This proves to be Zeikfried's future downfall.
Throughout the dungeon, the demon wannabe Zed attempts to stall the trio while Zeikfried attempts to locate Ka Dingel. Zed is fooled easily and thoroughly, and not even the golem Diablo is able to stop the trio, forcing their way in to fight Zeikfried for the first time in the game.
Following Zeikfried's defeat, he orders Zed to turn the generators to 200% output, creating a small black hole in the room. None are spared from the black hole, as Rudy, Jack, and Cecilia are transported close to Adlehyde, Zed is transported to Saint Centour, and Zeikfried is sent to the underwater ruins of the Photosphere.
This is where Rudy loses his ARM trying to save Jack and Cecilia from Zeikfried, and we also learn that Rudy is the Holmcross, made from living metal, like the demons.