If I cast Death from Below and the target gets gusted, can I still replay from trash?

No, you cannot use the recast effect of Death from Below if the target is no longer on the battlefield when the spell resolves.

According to RiftJudge FAQ #9400 and FAQ #9402, here is the sequence of events when your target is removed (in this case, by Gust):

Sequence of Events

1. Play: You play Death from Below, choosing a target unit at a battlefield.
2. Response: An opponent plays Gust (a [Reaction]), returning your target unit to their hand.
3. Resolution:
   • When Death from Below attempts to resolve, it checks its target. Because the unit is no longer on the board, the target is illegal (Rule 359.3.e.2).
   • The "Kill a unit" instruction fails to execute because the target is illegal (Rule 359.3.e.5).
   • The spell then attempts to check the target's Might to evaluate the condition: "if it had 3 [S] or less." Because the target is no longer on the board, its Might is treated as "null" (Rule 359.3.e.12).
   • Since the condition returns "null," the "you may play this from your trash" effect does not trigger.

The spell resolves, but because it fails to perform its primary instruction and the conditional check returns "null," you are unable to play the card from your trash.