GUM based pilots are prohibited from picking up trips from any other base or flying trips in the mainland. Recently there were trips that went HNL to SFO or LAX but never continued into the states. Also, non GUM based pilots can not pick up GUM trips. You could bid the 777 out of SFO and get trips that pass through GUM to HKG or take the plane to China and deadhead to GUM to get a feel for the island. With a new contract presumably coming soon this limitation could possibly change but it's unlikely.