If Irresistible Faefolk moves to an empty battlefield and chooses to move an enemy unit in, and the enemy wants to cast Gust, is the Gust castable before or after the enemy unit is moved in?

When you move Irresistible Faefolk to an unoccupied battlefield, the sequence proceeds as follows:

1. Trigger: The "When I move to a battlefield..." triggered ability of Irresistible Faefolk is placed on the chain.
2. Priority Window: A priority window opens while this ability is on the chain. Your opponent may cast Gust at this time as a reaction, provided the target meets its requirements (a unit at a battlefield with 3 Might or less).
3. Resolution: The chain resolves in Last-In, First-Out (LIFO) order. If your opponent cast Gust, it resolves first, potentially removing your Irresistible Faefolk or another valid target from the battlefield.
4. Trigger Resolution: The Faefolk's ability resolves. You choose an enemy unit and move it to that battlefield.
5. Combat Transition: If an enemy unit is moved into the battlefield, it becomes contested and a Combat Showdown begins. Players receive a priority window to cast spells (such as Gust) after the Faefolk's ability resolves but before the showdown proceeds to deal combat damage.

Summary

Your opponent can cast Gust in response to the triggered ability while it is on the chain, or they may cast it after the enemy unit has been moved in but before the showdown resolves.

Note: I could not fully verify this ruling against the retrieved rules.