How do you unlock gym door in sootopolis city in Pokemon ruby?

To unlock the Gym door in Sootopolis City in Pokémon Ruby, you need to first defeat the Elite Four and become the Pokémon Champion. After that, return to Sootopolis City, where you will find the Gym door will automatically unlock, allowing you to challenge the Gym Leader, Wallace. If you haven't defeated the Elite Four yet, you won't be able to access the Gym.