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 later versions of the event.
In the Heart of the Ruins update, when examining an Ancient Gateway with a key inserted into it, the character said: "The lights reveal ancient text: Come Back Next Update."
When first released, the Ancient Gateway had black and yellow tapes all over it, to signalize it was still a work in progress feature.
When the Gateway was first released, various materials were able to be socketed into it, such as the Iridescent Gem. Picking it back out produced an Ancient Key, regardless of what went in.
WX-78's quote, "ERROR 502" references the actual Error 502 which means "bad gateway error".
The Gateway is featured in the What Lies Beyond? video clip, for a brief moment revealing a silhouette that is suspected to be Winona.
The "Ancient Gateway" found in the Forge seems to also be called the Molten Gateway in the game generation description. It is unknown if this was its original name before being changed, or if it is what the structure was meant to be called later on and the in-game Gateway was simply not renamed as well.