Does equipping a Gear start a chain?

Yes, equipping a gear starts a chain. Equip is an activated ability (Rule 818.1), and like all activated abilities, activating it places the equip ability on the chain (Rule 333). This opens a window for your opponent to respond with Reaction cards before the equip resolves.

Here's the sequence:

1. Activation — You pay the equip cost and target a unit you control.
2. Chain starts — The equip ability is placed on the chain as a pending item.
3. Priority passes — The game state becomes Closed. Your opponent gets the first chance to play Reactions (e.g., a kill spell targeting the unit you're equipping).
4. Resolution — If the chain resolves and the target is still legal, the gear attaches to the unit.

Important caveat: If your opponent removes the target unit before the equip ability resolves, the equip fails — its target is no longer legal (FAQ #634).