If I cast Gust in response to Mirror Image, do they still get the token or does Mirror Image not resolve?

Yes, your opponent still gets the Reflection token.

In Riftbound, spells and abilities do not "fizzle" or vanish just because a target is removed. According to Rule 055 and Rule 359.3.e.11, players must execute as much of a card's instructions as possible, skipping only the impossible parts.

The Sequence of Events:

1. Opponent plays Mirror Image: They target a unit and place the spell on the chain.
2. You play Gust (as a Reaction): You target the unit chosen by Mirror Image and place Gust on top of the chain.
3. Chain Resolution (LIFO - Last-In, First-Out):
   • Gust resolves first: The targeted unit is returned to its owner's hand. It is no longer on the battlefield.
   • Mirror Image resolves second: The spell attempts to follow its instructions in order:
     1. "Play a ready Reflection unit token to your base": This is successful. A 0 [M] Reflection token enters play at your opponent's base.
     2. "Then do this: It becomes a copy of that unit": Because the original unit was returned to the hand by your Gust, it is no longer a valid target to be copied. This instruction cannot be followed, so it is ignored (Rule 359.3.e.6).
     3. "Give it [Temporary]": The Reflection token is given the [Temporary] keyword.

Final Result:

Your opponent will have a 0 Might Reflection token on the board with the [Temporary] keyword. It will not copy the stats or rules text of the unit you bounced, as that unit was no longer available to be copied when the second step of Mirror Image resolved.