The Gear Chamber is a room inside of the Old Clockworks in Luigi's Mansion: Dark Moon. It is only accessible by taking the door on the right inside of the Service Elevator. This is a large room with two floors.
The first floor has several gears turning a conveyor belt, as well as operating the large scale sin the middle. A Greenie blocks a hole near the scales; getting rid of it allows Luigi to look through it and spot another Greenie and a Strong Sneaker trying to hide the rotor. The door on the left takes Luigi back to the Service Elevator, while the door on the right leads to the Storage Room. On Floor B1, there is a large area of sand along with several crates; the door to the left leads to Floor B1 of the Service Elevator, while the door on the right leads to Floor B1 of the Storage Room.
During the mission Piece at Last, Luigi must go through the Gear Chamber to reach the Toad in the Synchronization Room; the scales that take Luigi downward will become temporarily stuck afterward, and the Strong Sneaker with the rotor disappears into the Storage Room. After rescuing Toad, Luigi is attacked by the Strong Sneaker upon returning to the Chamber, forcing him to fight several waves of ghosts: three normal Greenies, three Greenies with shovels, three Greenies with paranormal shields, three Mummy Greenies, and finally the Strong Sneaker. Once all ghosts are captured, the scales will return to normal, and Luigi must shoot Toad into the right scale on the first floor to tilt the elevator and allows access to the Storage Room. Once done inside the room, Luigi must go through the conveyor belt behind the scale to go back to the Service Elevator room and send the Toad assistant back to Professor E. Gadd.
A hidden Bonus Challenge can also be accessed during the mission; on the right side of Floor B1 in the Gear Chamber, there are several stacks of crates and a gap in between. Shining the Dark-Light Device on this gap reveals a missing piece of wood. After capturing the Spirit Ball, Luigi can walk across the plank to find a door shaking violently, which grants access to the Challenge. Once inside, he has to collect eight Red Coins in fifteen seconds; doing so will cause a Treasure Chest with a ruby to appear.
In Outlandish Interruption, stepping on the lift on the left will make it go up. There is also a valve to the south; turning this valve will tilt the lift, allowing access to the storage room. Nothing of importance happens here in this mission, however.