If an enemy unit flashes in response to Gust, do you need to retarget Gust to your own unit at the battlefield?

Ruling: No, you do not need to retarget Gust to your own unit. When Gust resolves, it whiffs and resolves to no effect because its target is no longer legal (not at the battlefield).

Sequence:

• Targeting happens when the spell is put on the chain
• The target cannot change after that point
• If the target flashes away, it is no longer at the battlefield
• When Gust resolves, it has no legal target and resolves to no effect

Nuances:

• This is different from Firestorm's reflexive trigger behavior