Into which valley will God gather all nations for judgment in Joel 3?

Answer: Jehoshaphat

God will gather all the nations to the Valley of Jehoshaphat and sit there to judge them for scattering His people.

“I will gather all the nations and bring them down to the Valley of Jehoshaphat. There I will enter into judgment against them concerning My people, My inheritance, Israel, whom they have scattered among the nations as they divided up My land.”— Joel 3:2 (BSB)