Does equipping start a chain?

Yes, equipping starts a chain.

According to RiftJudge FAQ #731 and Rule 818.1, Equip is an activated ability. When you activate an equip ability, it is placed on the chain, which allows players the opportunity to play reactions before it resolves.

The Sequence for Equipping:

1. Activation: The player pays the equip cost and targets a unit they control.
2. Chain Starts: The equip ability is placed on the chain.
3. Priority/Reaction: The state becomes closed, and players (starting with the opponent) may play [REACTION] cards (such as removal spells) in response.
4. Resolution: If the chain resolves and the ability is successful, the gear attaches to the chosen unit.

Important Nuance: As noted in RiftJudge FAQ #634, if an opponent plays a reaction that removes the target unit from the board before the equip ability resolves, the equip ability will fail to resolve because its target is no longer legal.