If Irelia Fervent (4 might, grows by 1 when chosen) is at 3 might and targeted by Gust (which targets units with 3 or less might), does Gust's requirement get checked on resolution, causing it to fail after Irelia grows to 4 might?

Ruling: Yes, Gust's requirement is checked upon resolution. When you choose Irelia with Gust, her trigger intercepts the chain and resolves first, bringing her to 4 might. Then when Gust resolves, the target is no longer valid so it whiffs (resolves but has no effect on the board).

Sequence:

• Gust is cast targeting Irelia at 3 might
• Irelia's trigger intercepts the chain and resolves first
• Irelia grows to 4 might
• Gust resolves but the target is no longer valid
• Gust whiffs (resolves with no effect)

Nuances:

• Spells never fizzle in Riftbound, they always resolve unless countered and follow DAMAYC (do as much as you can)
• All spells and abilities check their requirements upon resolution
• If opponent casts Gust on Irelia, there is no trigger so it goes through normally