After opening the door in the Rune-Warded Gate of Segrummar, you will be given the collection side quest to find more gears. The location of each one is shown below!
Some gears are only obtainable after opening other Sacrificial Gates of Segrummar.
Gears are sometimes on different elevations. If you can't see it, look up or down.
Although rare, a few gears require unusual amounts of platforming to reach. When you find it, trace back the path you will need to follow to reach it.
Finally, some gears can only be reached by completing expeditions. These can be done at the Legion of the Dead camp. Once you do one, a new bridge or pathway will open near the expedition site, which can be viewed on the map.
An important note, gear 7 and 8 can only be obtained after completing the expeditions in this area. This allows bridges to be built, which will let you reach the gears.