Opponent death from below targets a spirit in a battlefield might 2, I have a 7 might unit in different battlefield, what happens if I gust the spirit?

Because the target of your opponent's Death from Below is removed from the battlefield before the spell resolves, the spell will fail to resolve its primary instruction and will not trigger its recast effect.

According to RiftJudge FAQ #10423 and #9402, here is the sequence of events:

Sequence of Events

1. Play: Your opponent plays Death from Below and targets the Spirit (a unit on the battlefield).
2. Response: You play Gust in reaction, targeting the Spirit. Gust is placed on top of the chain.
3. Resolution (LIFO):
   • Resolution of Gust: The Spirit is returned to your opponent's hand.
   • Resolution of Death from Below: The spell attempts to resolve, but checks its target. Because the unit is no longer on the battlefield, the target is now illegal (Rule 359.3.e.2).
   • Partial Resolution/Failure: The primary instruction ("Kill a unit") cannot be executed because the target is illegal. The spell then attempts to evaluate the condition "if it had 3 [S] or less." Since the target is no longer on the board, its Might is treated as "null" (Rule 359.3.e.12).
   • Result: Because the condition cannot be evaluated, the "you may play this from your trash" effect does not trigger.

The spell resolves, but because the target is illegal, it fails to perform its primary instruction and cannot fulfill the requirement needed to trigger the recast effect. Your 7 might unit remains safely on its battlefield.