If I steal Guardian Angel with Akshan, equip it to my Lucian with Weapon Master, and Akshan dies, does my opponent get the Guardian Angel back?

Yes, your opponent regains control of the Guardian Angel when your Akshan leaves the board.

According to RiftJudge FAQs #9043 and #9151, the control-change effect created by Akshan is tied specifically to that unit. When Akshan leaves the board, that effect ceases to apply, and control of the equipment reverts to its original owner.

Here is the breakdown of the interaction:

1. Stolen Control: When you play Akshan and use his ability to steal the Guardian Angel, you gain control of that gear for as long as Akshan remains in play.
2. Attachment: Using a Weaponmaster effect (such as from a card like Jax) to move the Guardian Angel to your Lucian is a valid move. The equipment remains attached to Lucian even if Akshan leaves the board.
3. Control Returns: Once Akshan leaves the board, your control-change effect ends. Control of the Guardian Angel immediately reverts to your opponent.
4. Ongoing State: The equipment does not automatically detach or return to your opponent's hand/base just because control changed; it stays attached to your Lucian. However, because your opponent now controls it again, they may use their own card effects (such as their own Weaponmaster ability) to move the Guardian Angel to one of their own units at their convenience. You can no longer use the equipment's abilities while you do not control it.