Can I gust a traveling merchant to cancel the "discard draw" effect?

No, you cannot cancel the "discard 1, draw 1" effect by using Gust on Traveling Merchant.

According to the official RiftJudge FAQ (specifically FAQ #1997, #4090, and #6425), once the Traveling Merchant moves, its ability is triggered and added to the chain. Even if you use Gust to return the Merchant to its owner's hand in response, the ability is already on the chain and will still resolve.

Sequence of Events:

1. Trigger: Traveling Merchant moves to a battlefield, triggering its "discard 1, draw 1" ability.
2. Chain: The ability is added to the chain.
3. Response: You play Gust (a [Reaction]) in response to the triggered ability.
4. Resolution (LIFO): The chain resolves in reverse order.
   • Gust resolves first, returning the Traveling Merchant to its owner's hand.
   • The Traveling Merchant's triggered ability resolves next.
5. Effect: The player discards and draws as instructed (or as much as possible if their hand is empty/has few cards), regardless of the fact that the unit is no longer on the board.

As noted in the rules and FAQs, triggered abilities in Riftbound resolve independently of whether the source of the ability remains on the battlefield.