Can you play reactions to opponents playing units or gear, and can you react to activated abilities?

Ruling: You cannot play reactions to units or gear being played. However, you can play reactions to triggered abilities that result from units/gear being played, and you can play reactions to activated abilities.

Nuances:

• Units and gear become game objects after being finalized, not chain items
• Triggers and spells become chain items that can be reacted to
• The distinction is between the permanent itself (unreactable) and abilities it creates (reactable)