Can you play Acceptable Losses to kill an enemy Gear if you don't own any?

Yes, you can play Acceptable Losses even if you do not control any gear, as long as your opponent does.

According to RiftJudge FAQs #4344, #9554, and #10428:

• Acceptable Losses does not target.
• At the time of resolution, each player who controls gear will choose one to kill.
• If you control no gear when the spell resolves, the instruction for you to kill a gear simply cannot be performed and is ignored (see Rule 359.3.e.6).