If I play Switcheroo and the enemy Gusts my unit in reaction, does the enemy still get reduced Might?

To determine the outcome of this interaction, we must analyze how the chain resolves when a target of a spell becomes invalid before that spell executes.

The Sequence of Events

1. Your Action: You play Switcheroo, targeting your unit (Unit A) and an enemy unit (Unit B) at the same battlefield. This creates a "Closed State" on the chain.
2. Opponent's Response: Your opponent plays Gust, targeting your unit (Unit A), assuming it has 3 Might or less.
3. The Chain (Bottom to Top):
   • Switcheroo (Bottom)
   • Gust (Top)

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

According to Rule 359.3.e.7 and the logic established in Rule 359.3.e.12, here is how the resolution proceeds:

1. Gust Resolves: Since your unit (Unit A) is a legal target for Gust at the time of resolution, the Gust effect executes, returning your unit to your hand.
2. Switcheroo Resolves: Switcheroo now attempts to execute its instruction: "Swap the Might of two units at the same battlefield this turn."

Does the Enemy Unit Get Reduced/Changed Might?

No, the enemy unit will not have its Might changed.

According to Rule 359.3.e.12, if a spell checks information about a target that is no longer legal—or whose location/status has changed such that the information is unavailable—that check returns "null," and calculations based on it are ignored.

Furthermore, Rule 359.3.e.7 states that if all of an instruction's targets become invalid or unavailable by the time it executes, that instruction will not execute. Even if Switcheroo is technically only targeting one remaining unit (Unit B), the instruction "Swap the Might of two units" relies on the existence and Might values of both targets to calculate the result. Because Unit A is no longer on the battlefield, the "swap" calculation cannot be performed. Consequently, the effect fails to execute.

This is consistent with the Baited Hook example in Rule 359.3.e.12, where a spell requiring two targets (or specific information about two targets) fails to produce an effect if one target is removed, because the comparison required to perform the action becomes impossible.

Note: This is my interpretation based on the mechanics of target invalidation and calculation requirements found in Rule 359.3.e.12 and Rule 359.3.e.7. No official FAQ exists for this specific interaction, but it follows the established rules for resolving instructions dependent on multiple targets.