If I ambush a unit on Starspring and try to trigger it, but that unit gets gusted, can I play that unit again and still trigger Starspring?

No. Star Spring triggers only "the first time a player plays a non-token unit here each turn," and that first-time trigger is already used up for you this turn — Gusting your ambush unit doesn't reset it.

The Gust also doesn't cancel the trigger that's already on the chain: Star Spring's ability resolves on its own to move another unit you control there to base, even if the unit that triggered it was bounced. Replaying that unit to Star Spring is a second play by you this turn, so the ability won't trigger again (Rule 383.3.e.1).

One caveat: if instead of performing the trigger you had declined it during finalization (the "you may"), it's considered to have not triggered, and a later play of a non-token unit there could trigger it (Rule 383.3.e.2.b).