petalburg city gym. That is the one where you 'Dad' is the gym leader. hope this helped.
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In Pokemon Red, the fifth badge is possessed by the Poison-Type Koga. The badge is named Soul Badge. It lets you use Surf outside of Battle.
at the gym
Go back to your dads gym in petalburg
Go to the Pokemon league.
You need to defeat Koga in Fuchsia City to get the 5th badge, the Soul Badge.