Can you react to Gust with a hidden Fight or Flight to return the unit to base?

Ruling: Yes, you can react to Gust with a hidden Fight or Flight if the unit is at the battlefield where Fight or Flight is hidden.

Nuances:

• Hidden cards can only target cards at the battlefield where they are hidden
• Hidden cards can be played at reaction speed (any time a reaction can be played)
• Hidden cards can only be used starting from the turn after you hid them
• Hidden cards can only be placed on battlefields you control
• If you lose control of a battlefield, hidden cards there get trashed immediately
• Hidden cards cannot be placed on the base
• Hidden cards cannot be moved between battlefields