Can Grand Stratagem be played as a reaction to Challenge during a showdown?

Ruling: No, Challenge and Grand Stratagem are both actions, so they cannot be played in response to other cards.

Sequence: (How a showdown with Challenge works)

• You move units and start a showdown
• You have focus to start a chain
• You pass
• Opponent has focus and plays Challenge then passes priority
• You can play reactions or pass
• Challenge resolves

Nuances:

• Grand Stratagem cannot be played as a reaction because it is an action, not a reaction