A 3 might unit is moving from base to Trifarian Warcamp. Can opponent play Gust at reaction speed?
No, your opponent cannot play Gust in response to the move itself.
According to Rule 441.3.c, "Moving does not use the Chain, nor is it able to be Reacted to." Because the act of a unit moving from base to a battlefield does not trigger any abilities and does not use the chain, there is no window of priority for your opponent to play a reaction card like Gust while the unit is in transit or upon its arrival.
Why this happens:
- No Chain: Moving is a game action that happens instantaneously (Rule 441.3.c). It does not create an item on the chain, so there is no "Closed State" for your opponent to enter.
- Continuous Buff: As soon as your unit completes its move and enters the Trifarian War Camp, the battlefield's passive ability (+1 Might) applies immediately (Rule 710/FAQ #2885). Your 3-Might unit becomes a 4-Might unit the instant it arrives.
- Target Legality: Even if your opponent were to gain priority later (for example, if you performed an action that created a new chain), Gust would check the unit's might at that time. Since it is now 4 Might, it is an illegal target for Gust.
In short, movement cannot be reacted to, and the battlefield buff effectively protects the unit from Gust by increasing its might as it lands.