If you apply the GUC to the JFK-LAX leg and clear into PS (or D1) for that leg then that GUC is considered "used." The GUC should, if coded properly, still allow you to be on the waitlist for PS/D1 on the LAX-AKL leg, and if nothing goes wrong such as IROPs then that one GUC could get you all the way to AKL with whatever upgrades clear along the way. I would say the only risk here is that if you don't clear on the LAX-AKL leg, just know the GUC is gone on the shorter leg (though JFK-LAX is long enough it could be worth it - the real issue becomes when the GUC gets burned on a super short domestic hop such as SFO-LAX, etc.). All in all I'd say it's worth it to apply to the whole journey unless you foresee another international trip to use it on. If you do call and have it applied, make sure the agent applies it to BOTH segments - and call back a second time to verify things are waitlisted properly.