Can I cast Gust on an opponent's Sneaky Deckhand as it enters a battlefield I control on their turn?

Ruling: Yes, you can cast Gust on the Sneaky Deckhand. When a unit exists at a battlefield its player doesn't control, it will always initiate a Showdown, which gives you an opportunity to cast Gust after any Actions your opponent may make before giving Focus to you.

Sequence:

• Opponent's unit enters the battlefield you control
• A Showdown is initiated
• Opponent may take Actions
• Focus passes to you
• You can cast Gust on the unit