Can I react to my opponent equipping a gear to a unit?

Yes, you can react if your opponent attaches a gear using an Equip ability.

According to Rule 818.1, Equip is an activated ability. Activating an ability follows the same process as playing a card, which includes adding a pending item to the chain and creating a Closed State (Rule 150.2.a). Because the ability is placed on the chain, it creates a window for players to respond with reactions before the ability resolves.

The Sequence:

1. Activation: Your opponent activates the Equip ability on their gear, choosing a target unit.
2. Chain Starts: The Equip ability is placed on the chain.
3. Priority: The state becomes Closed. You now have the opportunity to play a [Reaction] card or activate a [Reaction] ability.
4. Resolution: If you play a reaction (such as a spell to remove the target unit), your reaction will resolve first (LIFO - Last In, First Out). If the target unit is removed or the ability is countered, the Equip ability will fail to resolve on that unit.

Important Note: This applies specifically to the Equip activated ability. If a gear is simply being played to the board (not using an Equip ability to attach), it enters the chain and resolves immediately, skipping the opportunity for reactions.