How do you get to the 5 gym on Pokemon Diamond?

1. Defeat the Gym Leader in Canalave City. The Canalave City Gym is located on the west side of town. The Gym Leader is Byron, who uses Steel-type Pokémon.

2. Go to the Battle Zone. The Battle Zone is located on the northern part of Route 228.

3. Talk to the man in the Battle Zone. He will give you a VS Seeker.

4. Use the VS Seeker to find the 5th Gym Leader. The 5th Gym Leader is Volkner, who uses Electric-type Pokémon. He can be found at the Sunyshore City Gym, which is located in the northwestern part of Sunyshore City.