Does attaching Gear create a chain that can be reacted to?

Yes, equipping/attaching a gear via its Equip ability creates a chain and can be reacted to. This is a confirmed ruling (FAQ #731).

Equip is an activated ability (Rule 818.1). Activating it follows the same process as playing a card — you pay the cost, choose a target, the ability goes on the chain as a pending item, and the game state becomes Closed. Your opponent can then play Reaction cards or abilities before it resolves (Rule 150.2.a, Rule 377.3).

The one nuance: if the gear is being played from hand with Quick-Draw (Rule 819), the gear card itself resolves immediately without giving opponents a window (same as any unit/gear entering play). However, Quick-Draw's "When you play this, attach it to a unit you control" is a triggered ability that does go on the chain after the gear enters play — and that trigger can be responded to.