I can also confirm that he drops the gearmost core as well (I am aware this is COMMANDER and not CAPTAIN gearmost). Same 80 HP/Mana aug captain gearmost can drop. Had one drop for me off him 10 minutes ago and I double checked the name before posting.
This mob spawns on the roof in Beza. I've seen him once wandering around the outside of the shed up there, and I've also seen him as a static mob INSIDE the shed (the screenshot is of him inside the shed).
Commander Gearmost has an abnormally large aggro range, so be careful. When I camped him, I would set my casters up on the little thin ledge in front of the big window into the shed to avoid aggroing anything, and have the melees around the corner fighting. Mobs don't aggro through the glass.
Our tank on him was a level 80 monk with max defensives, about 3400 AC, 8% dodge mod, improved dodge III, improved block III, 94 avoidance, and 17% shielding. Commander Gearmost's max hit was 2541. He is slowable. He does flurry, which increases his DPS a good bit. Like most named, he DOES see through invis, he DOES summon, and DOESN'T run at low health. No special effects, no adds, nothing special about him. He put out 1059 DPS against our tank.
Gearmost is pretty straightforward mob. Dropped a Glossy Silk Hat the first time I killed him and a Scratched Hide Cowl the second time I killed him.