Can I gust the reflection token after he discards if my opponent conquers and creates a reflection of ruined Rex? Would this trigger deathknell?

Yes, you can Gust the Reflection token after your opponent discards, but doing so will not trigger its Deathknell ability.

Why you can Gust the Reflection

When your opponent conquers and uses their leader ability to create a Reflection token (which then becomes a copy of Ruined Rex), that ability process uses the chain. Because Gust is a [Reaction], you can play it while the chain is still active (the "Closed" state) to return the token to its owner's hand.

Why the Deathknell will not trigger

The Deathknell keyword is defined as a triggered ability that occurs specifically when a permanent is killed and sent to the trash (Rule 808.1.d).

• The Cause: Gust explicitly instructs you to "Return a unit... to its owner's hand."
• The Result: Because the token is moved to the hand and not the trash, the condition required for the Deathknell to trigger is never met (Rule 808.1.d.1; see also RiftJudge FAQ #10059 regarding similar interactions).

Summary of Interaction

1. Chain: The Reflection token enters the battlefield.
2. Reaction: You play Gust, targeting the Reflection token.
3. Resolution (LIFO): Gust resolves first, returning the token to your opponent's hand.
4. Deathknell: Because the token was recalled to the hand rather than being sent to the trash, the Deathknell ability never triggers.