Guild War Shop becomes available at team level 30. You can access it by tapping the Shop tab in the bottom-left corner of the Guild War screen:
You can also select the Guild War Shop tab when you open any other Shop:
Here, you can spend Guild War Trophies to purchase unique Champion's skins, Titan Potions, Chests of different rarity, gold, and Soul Stones of Jhu, Ziri, Nebula, and K'arkh.
This shop cannot be refreshed, and the goods are always the same.