If my opponent targets my 2 Might unit with Gust, and I respond by buffing it to 4 Might with Discipline, does Gust still return my unit to hand?

No, Gust will not return your unit to hand. The game checks if targets are legal both at activation (when the spell is played) and at resolution (when the spell effect applies). When Gust is played targeting your 2 Might unit, the target is legal at activation. However, when Discipline resolves first and increases your unit to 4 Might, the target becomes illegal because it now exceeds Gust's targeting requirement of 3 or less Might. When Gust resolves, it checks legality again, sees the unit now has 4 Might, and therefore resolves but has no effect.