Can you use Gust (or any 'react' response) immediately after a unit is played, or in response to unit movement?

Ruling: You cannot react directly to a unit being played or to movement itself. You can only react if the unit or movement triggers an ability.

Sequence:

• When a unit is played or moves, check if it has a triggered ability (e.g., "When you play me..." or "When I move...")
• If there is a triggered ability, you can react to that ability
• If you react to kill the unit (like Harnessed Dragon), the triggered ability still resolves

Nuances:

• You are reacting to the triggered ability, not to the unit or movement action itself
• Even if you kill a unit in response to its triggered ability, that ability still resolves (example: Harnessed Dragon's ability to kill your unit will still happen)