The Guild Championship Shop becomes available at team level 30. You can access it by tapping Merchant on the Global Championship main screen:
You can also select the Guild Championship Shop tab when you open any other Shop:
Here, you can exchange Global Championship trophies for Guild War trophies and purchase Boosters, unique avatars, avatar frames, and Advanced Teleports for Realm.
This shop cannot be refreshed, and the goods are always the same.