If you set Voice Services -> Auto Attendant -> PrimaryLine to PSTN Line, then calls made through the autoattendant will go out on the PSTN line by default. After dialing 2 for an outgoing call (and entering PIN if required), you can dial **1 + number to send the call by GV.
Without the above setting, AA calls will use GV by default, but the caller can use **8 to send the call on the PSTN line.