Its in Pewter city.
Brock's gym is in Pewter City.
where can i find the second gym in Pokemon platinum
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In the gym!
in his gym
In the goldenrod gym.
In her Gym
You will find the key to the Gym of Cinnabar Island, From the Pokemon Mansion that is on the same island.
Find the secret key in the Pokemon Mansion,( Next to the gym)
you find her in the old chateau
You find him in the last Gym in Sootoolis City.He is the last Gym leader
You can find the 2nd gym in the south part of Eterna City.