Go to Wally's House in Petalburg City, and one of his relatives will give you the Surf HM, which you can use outside of battle with the Balance Badge. Once you've taught a Pokémon how to Surf, return to Mauville City and head due east. Cross the water and then follow the path north from there to reach Fortree City, where the sixth gym is.
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Try his perents house
In the 8th gym. he is the gym leader
The gym leader is not at the gym and that is why we cannot enter the gym.
The 6th Gym Leader In Pokemon Ruby Destiny is in Down Pour City
Beat the Gym Leader