To play Acceptable Losses, do I need to have a gear to play it, or can I simply play it when only my opponent has a gear?

No, you do not need to control your own gear on the field to play Acceptable Losses.

According to RiftJudge FAQ #4344 and #9554, you can play Acceptable Losses even if you do not control any gear, as long as your opponent does. The card does not target; at resolution, each player who controls gear will choose one to kill.