Can you Gust a Pyke Dockside Butcher after the opponent plays it from hidden and pays the additional red rune cost?

Yes, you can Gust Pyke, Dockside Butcher after your opponent plays it from hidden and pays its additional cost.

Here is the breakdown of the sequence and why this works:

1. Playing from Hidden

According to RiftJudge FAQ #10539 and Rule 811.1.b, when a player plays a unit from Hidden, the unit enters play. This action uses the chain. The unit's "When you play me" triggered ability—which checks if the additional cost was paid—is also placed on the chain.

2. The Sequence

1. Playing Pyke: The opponent plays Pyke from hidden. Pyke enters the battlefield.
2. Triggered Ability: Because the opponent paid the additional cost ([C]), Pyke’s "When you play me" triggered ability is placed on the chain to grant him the +2 [S] and ready him.
3. Reaction Window: Because there is now an item on the chain (the triggered ability), the game state is Closed. This creates a window for you to play [Reaction] speed cards.
4. Playing Gust: You play Gust targeting the newly played Pyke. Gust is placed on the chain on top of the Pyke trigger.

3. Resolution (LIFO)

According to Rule 404.1 (and the LIFO principles defined in the showdown flow), the chain resolves in reverse order:

1. Gust resolves: Pyke is returned to its owner's hand.
2. Pyke's trigger resolves: The ability attempts to resolve, but since Pyke is no longer at the battlefield (he is back in the owner's hand), the ability fails to find its target/source and does not grant the +2 [S] or ready him.

Summary: Because Pyke's triggered ability uses the chain, you gain a priority window to respond with Gust before that ability resolves. Returning Pyke to the hand removes him from the board, causing his triggered ability to fail.