Can you use a card like Gust to remove a unit from battlefield B when reacting during a showdown happening on battlefield A?

Ruling: Yes, you can use Gust or any other spell at Battlefield A during a showdown at Battlefield B. Targeting restrictions only apply to Hidden cards.

Nuances:

• If a card like Hidden Blade was Hidden at battlefield B during a showdown at battlefield B, it cannot target battlefield A when reacted
• The targeting restriction applies specifically to Hidden cards only