Does reacting with Discipline to Gust fizzles Gust if a unit is now 5 might?

No, Gust does not "fizzle."

In Riftbound, spells do not fizzle in the traditional sense. When a spell resolves, it resolves as much as possible, even if its targets have become illegal.

According to RiftJudge FAQ #36 and Rule 359.3.e.1, if you react to Gust by playing Discipline to increase your unit's might above 3, the target becomes illegal for Gust. When Gust resolves, it checks the target's legality again:

1. It sees the target now has 5 Might (making it an illegal target for the "3 might or less" requirement).
2. It resolves without having any effect on that unit.
3. The spell then proceeds to the trash.

As stated in the RiftJudge FAQ #5530 (which clarifies that spells do not fizzle), the spell "resolves but does nothing" to the now-illegal target.