When I play Gust on an enemy Unit and the enemy buffs the unit above 3 Might, does Gust still count as Played and trigger the Ravenbloom Student?

Yes. Gust still counts as played and will trigger Ravenbloom Student.

Here's the sequence:

1. You play Gust targeting a unit with 3 Might or less — it goes on the Chain.
2. Your opponent responds by buffing that unit above 3 Might — that buff resolves first (top of chain).
3. Gust then resolves. It checks its target and finds it no longer meets "3 Might or less," so the instruction to return it to hand is ignored.
4. However, per Rule 359.3.e.10, a spell that resolves with no effect because its targets became invalid is still considered played.

Since Gust resolved successfully (it just did nothing to the board), Ravenbloom Student's "When you play a spell" trigger will activate and give it +1 Might.