Can you play a reaction if my opponent attaches a Gear?

Yes, you can play a reaction 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, Rule 386–389). 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 (see FAQ 5130 and 8333).