Originally Posted by
SPN Lifer
If you start at T3, either through parking or via BART & Air Train, you only have to walk one way.
BART terminates in the international terminal ~100 yards from the G gate security check. If that's how you're arriving there's no reason to go to T3 for a flight out of G gates.
I always walk from BART to T3 security instead of taking the AirTrain, it's only 5-7 minutes. But that's a separate topic.