Can a gear be at a battlefield alone, and specifically can a hidden Zhonya's Hourglass be used to save itself when the only unit at that battlefield is targeted by Charm?

Ruling: A gear cannot be at a battlefield alone. If a gear is at a battlefield without a unit, it is immediately recalled to base. You can reveal a hidden Zhonya's Hourglass before the unit is moved by Charm to save it from being trashed.

Sequence:

• Opponent plays Charm, targeting your only unit at the battlefield where Zhonya's is hidden
• Opponent passes priority
• You reveal and play Zhonya's
• Zhonya's immediately resolves
• A cleanup happens and recalls Zhonya's to base (since no unit remains)
• Priority passes back and forth
• Charm resolves and moves your unit

Nuances:

• The gear must be revealed before the unit is moved to avoid being trashed while hidden
• Permanents cannot be reacted to - once they enter the chain, they immediately resolve
• There was discussion about a rules ambiguity regarding permanents with reaction speed in the middle of a chain, but the intended behavior is that permanents resolve immediately