You can easy take Bart from airport to downtown SF and GG bridge is close by.
Admittedly, 8 hrs is a long time to hang around an airport, and, although theoretically you could get to San Francisco Bay and the bridge in a couple of hrs, it would be a rather expensive, and time consuming trip. Furthermore, regarding storage lockers, due to the incidence in the past, of people placing dangerous devices in them, few airports have such a facility . Anyway, enquire on your arrival, but don't jeapodise your connection flight.
Hell No The GG bride is not down the street and to your left.
Not sure about holding luggage but if your flight is not international you can easily leave and come back in that amount of time
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