When you reveal Evelyn, can you gust it, what happens?

Yes, you can use Gust to react to Evelynn, Entrancing when she is played from a hidden state.

According to Rule 811.6, cards that are Hidden gain the [Reaction] keyword while facedown or when played from facedown, allowing you to play them at reaction speed. When you reveal (play) Evelynn, her "When you play me from face down" ability triggers, creating a chain. Because this ability uses the chain, you (or your opponent) can play Gust as a reaction to it.

Sequence of Events

1. Evelynn is played: You reveal Evelynn from her hidden state and place her on the battlefield. Her triggered ability ("When you play me from face down...") is placed on the chain.
2. Reaction: An opponent plays Gust targeting Evelynn, adding it to the chain above her ability.
3. Gust resolves (LIFO): Gust resolves first, returning Evelynn to your hand.
4. Evelynn's ability resolves: The ability attempts to resolve, but because Evelynn is no longer on the battlefield to serve as the destination for the enemy unit, the ability fails to move any unit.

Summary: By using Gust in response to Evelynn's reveal, you successfully return her to your hand and prevent her ability from moving an enemy unit. This interaction follows the established Riftbound rules for chain resolution (LIFO) and the requirement that a card must remain on the board to complete "move to my battlefield" effects (referencing RiftJudge FAQ 10352 regarding similar "move" triggers).