Find gear in Mountain Temple was a mission in Jak II. Jak was looking for answers regarding the Tomb of Mar, for which in turn Onin send Jak to collect three artifacts from the Mountain Temple, one of them was the Gear.
The PlayStation 3 and Vita versions of this mission are related to a trophy named "Just the Artifacts Ma'am", completing this one and the other two related artifact missions at the Mountain Temple will reward you with a bronze trophy.
A collection of three mission spread out over a single large area, each with its own linear path. When entering the mountain area and taking the elevator down; head forward and take the middle path past the new weak hoppers. Go down these floating platforms and a ram head will show up in the area down below. It attacks by charging so dodge sideways to avoid it, then turn around and shoot its only weak spot; its underbelly.
When you kill it a button appears, which when pressed, reveals some new platforms to climb on. Fight some manic heads to come to a difficult mountain climbing section, with a constant rain of big boulders. Often enough the camera blocks your view here making it more luck-based, although there are several safe spots in the area. Take it slow and only proceed when you are certain it's safe (preferably towards a spot you checked out before which is also safe).
After this it is smooth sailing, a couple more platform jumps and you are then good to go and grab the Gear. Another platform will take you back to the starting area.