If I use Possession on an enemy unit with gears attached to it, do I take control of the unit with the gears too?

No. You take control of the unit itself, but you do not gain control of the attached Gear.

The Possession spell targets a unit — only the top-most card changes control. The equipped Gear remains attached to that unit, and the original owner retains control of the Gear itself. While the Gear stays attached, you benefit from its effect text (e.g. stat boosts), but you cannot activate its abilities — including Equip to move it — because you don't control the gear.

If the possessed unit later dies or leaves the battlefield, the Gear detaches and is Recalled to the original owner's Base during the next Cleanup. (Per RiftJudge FAQ #10699 and #5310.)