If I gust a star crossed enemy unit while it's targeting my unit, does star cross fizzle?

No — Star-Crossed doesn't fizzle; it still resolves and returns your unit.

You Gust the enemy unit that Star-Crossed is targeting. Gust resolves first, returning that unit to hand. Since that unit has now moved to a non-board zone, it's no longer the same game object, so Star-Crossed's instruction for it is ignored (Rule 359.3.e.8). But your unit remains a valid target, so Star-Crossed still resolves and returns your unit to your hand.

There's no "fizzle" in Riftbound — spells resolve as much as possible, ignoring only instructions tied to illegal targets. Your only way out would have been to make your own unit illegal (e.g., moving it to a non-board zone) before Star-Crossed resolved. (FAQ #11166)