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 Forgeevent, the Gateway in the Forge dimension shows up as the starting point where players and their allies spawn at the 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, and is also called the Molten Gateway. It currently has no other function, but the game files reveal that it also has an active state, which might be used in later versions of the event.
In The Gorgeevent, the Gateway in the Elder Bog dimension shows up as the starting point where players spawn at the beginning of the game. The Elder Bog's Ancient Gateway is technically a separate structure sharing the same name as the one located in the Atrium, and is also called the Mossy Gateway. If the players win a round, an ending animation will play in which Mumsy gives a victory speech and the characters jump back into the reactivated portal.
When first released, the Ancient Gateway had black and yellow tapes all over it, to signalize it was still a work in progress feature. During this time, various Items were able to be socketed into it, such as the Iridescent Gem. Picking it back out always produced an Ancient Key. When examining an Ancient Gateway with a key inserted into it, the player character said: "The lights reveal ancient text: Come Back Next Update."
The Gateway is featured in the What Lies Beyond? video clip, for a brief moment revealing a silhouette of Winona before her official reveal.
The Ancient Gateway located in the Ruins area of the Constant is also called the Atrium Gate.