I would say that you should be back at the airport, in the security line, 1 hour prior to departure time. A little earlier if you wish to return to the WorldClub and have a drink there prior to boarding.
As for the checkpoint at the back of the parking garage, between G and C, depending on the time of day the line can be very short, or very long. And once you get through security you will be a long way from the international departure gates G1-G8. There is a Priority Line in the main terminal check-in area, reserved for FC, WBC, and Elites, which makes it a snap to clear security there and be much closer to the gates and the larger F-G WorldClub.