Can I weaponmaster a piece of gear back to my unit if the enemy Akshan has stolen it and equipped it to a different unit?

No, you cannot use Weaponmaster to take back a piece of gear that an enemy Akshan has stolen.

According to Rule 747.1.b, the Weaponmaster keyword allows you to choose an Equipment you control. When an enemy Akshan steals your gear, they gain control of that equipment for as long as their Akshan remains on the board. Because you no longer control the equipment, you cannot target it with your own Weaponmaster ability.