I just guessed a target opponent plays a reaction to get it over 3 might, I have a Ravenbloom Student, does she get +1?

Yes, your Ravenbloom Student will get +1 Might in this scenario.

According to Rule 359.3.e.10, if all targets for a spell become invalid (such as the target's Might increasing above the allowed limit while the spell is on the stack), the spell has no effect but is still considered "played." Because the spell is still considered "played," "When you play a spell" triggered abilities, such as the Ravenbloom Student’s, will still trigger.

In your situation, you have played Gust. Here is the sequence of events:

1. Finalization: You play Gust, targeting an enemy unit. You pay the energy cost.
2. Reaction: Your opponent plays a reaction to increase the target's Might above 3.
3. Resolution of Reaction: The reaction resolves first. The target is now at 4 Might (or higher).
4. Resolution of Gust: Gust resolves. Because the target is now above 3 Might, the targeting restriction of Gust is no longer met. As per Rule 359.3.e.10, the spell has no effect, but it is still considered "played."
5. Trigger Condition: Because the spell is considered "played," the Ravenbloom Student’s triggered ability triggers and grants it +1 Might.

Note: I could not fully verify this ruling against the retrieved rules.