Can I respond to the trigger of Frigid Jewel with Gust when my opponent goes to showdown with a 3 might unit and then draws a 2nd card?

Yes, you can respond to the Frigid Jewel trigger with Gust.

The Sequence

1. Triggered Ability: When your opponent draws their second card, Frigid Jewel's ability triggers. It is placed on the chain, creating a Closed State.
2. Priority: Because a triggered ability is on the chain, the state is closed, and you receive priority to play a [Reaction].
3. Your Response: You play Gust as a [Reaction], targeting your opponent's 3-might unit.

Resolution (LIFO - Last-In, First-Out)

According to Rule 359.3.e.7 and the Showdown Timing Flowchart, the chain resolves in reverse order:

1. Gust Resolves: It checks the target. Since the unit is currently at 3 might, it is a legal target. Gust returns the unit to your opponent's hand.
2. Frigid Jewel Resolves: It attempts to resolve its effect ("give a friendly unit +2 [S]"). Since the unit that was targeted is no longer on the battlefield, the ability cannot apply the bonus. It resolves with no effect.

This interaction is consistent with the established mechanics for responding to triggered abilities as confirmed by RiftJudge FAQ #10323 and Rule 383.