Gears can be found in almost any factory/industry location and especially Mechanist institutions. Their use predates (at least) the end of the last dynasty of Karath-din, as can be seen at the imperial colosseum in the TDP/TG mission The Lost City. Only in Thief II does the gear obtain a symbolic use as well as a mechanical one, a possible exception being the gear required to lower the cage containing The Builder's Chalice in St. Edgar's Church.
Notable gear items include the Plain Gear, Unstamped Gear, Diamond Gear, and the Locking Mechanism Gears.