Its only use is to have an Ancient Key to be placed in its socket, making the Atrium's Nightmare Cycle be locked at the peak state. It can be removed again, unlocking the Cycle from its peak. If a fully-constructed Odd Skeleton is revived near an activated Ancient Gateway, the fight against the Ancient Fuelweaver will begin. The Ancient Fuelweaver can deactivate the Atrium's Nightmare Cycle by removing the Ancient Key. If the Ancient Fuelweaver is killed it will trigger an explosion that will activate after three minutes, renewing the Ruins. Twenty days later, the Gateway will accept the Ancient Key to be placed again.
In the Forge event, the Gateway in the Forge dimension shows up as the starting point where players and their allies spawn at beginning of the game. The Forge's Ancient Gateway is technically a separate structure sharing the same name as the one located in the Atrium in normal DST. It currently has no other function, but the game files reveal that it also has an active state, which might be used in the official version of the event.