You can buy GCM passes from the npc near the fountain. You can do resource exchange for GCM passes with mob drops from Liaodong map. You can loot GCM dungeon chests for passes after you kill the final boss inside. You "might" be able to get GCM passes from CCD green. (Need verification).
If I remember correctly, CCD red drops regular SRS passes and superior GCM passes. Which would follow that CCD green drops regular GCM passes? I can not be certain at this point, therefore I wrote "(Need verification)", so Joe is aware.
Yes but you need to be around lv 90~100 otherwise items won't drop. CCD red drops relugar SRS passes and does not drop super GCM passes.