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In the sootopolis city gym.
In Pokemon Ruby, Wallace is the gym leader of Sootopolis City. When the player first arrives in the city, Wallace is not at the gym. He and Steven are north of the city. They take the player to the Cave of Origin where Groudon is encountered. After this battle, Wallace can be found and battled in the Sootopolis Gym.
This is because you have to go in the cave at the top of the town then the gym will open.
It is in Sootopolis City, I think, left of the gym. I forget which house.
Surf to Evergrand city through the victory road and defeat the Pokemon league