What happens if you gust a Reflection token after it has copied other unit?

It depends on what Might the Reflection token has after copying.

If the Reflection copied a unit with Might 3 or less, Gust can target it. The token is returned to its owner's hand — then it immediately ceases to exist per Rule 183.1, since tokens can't exist outside the board or chain. It is not killed, so any Deathknell on the copied unit's text does not trigger (FAQ #10484).

If the Reflection copied a unit with Might 4 or more, Gust cannot target it at all — Gust's targeting restriction requires "a unit at a battlefield with 3 might or less."

And on the off chance you're thinking of Gusting before the copy happens (reacting to the ability creating the token), that's a different scenario — covered in FAQ #10681 — where the token still enters play but stays a 0 [M] base token because its copy target vanished.