If I gust a unit that was star crossed, does star crossed still resolve?

Yes, Star-Crossed will still resolve.

When you use Gust to return a unit targeted by Star-Crossed to its owner's hand, that unit leaves the battlefield and enters a non-board zone. This makes the target "illegal" for the Star-Crossed spell because it is no longer the same game object as the one originally targeted (Rule 359.3.e.2, Rule 359.3.e.4).

However, Star-Crossed does not "fizzle" (fail entirely). According to Rule 359.3.e.8, if a spell has multiple targets and one becomes illegal, the spell proceeds to resolve as much as possible, ignoring only the instruction that involves the illegal target.

The Sequence

1. Star-Crossed is played: It targets one friendly unit and one enemy unit. Star-Crossed is added to the chain.
2. Gust is played in response: It targets the unit (which is a valid target for Gust). It is added to the top of the chain.
3. Chain State (Top to Bottom):
   • [Top] Gust
   • [Bottom] Star-Crossed
4. Resolution (LIFO - Last-In, First-Out):
   • Gust resolves first: The targeted unit is returned to its owner's hand.
   • Star-Crossed resolves: It checks for its targets.
     • If the target returned by Gust was the enemy unit, the instruction to return that unit is ignored. The spell proceeds to return your friendly unit to your hand.
     • If the target returned by Gust was your friendly unit, the instruction to return that unit is ignored. The spell proceeds to return the enemy unit to its owner's hand.

This is consistent with the mechanics of partial resolution for spells with multiple targets and the rulings found in RiftJudge FAQ #10990 and #10736.