1. Defeat the Elite Four: This is the first and most important step. You must defeat the Elite Four in the Pokémon League to unlock the eighth Gym.
2. Return to the Gym: After defeating the Elite Four, you will be able to return to the eighth Gym, which is located in Veilstone City.
The eighth Gym is led by Maylene, the Fighting-type Gym Leader.
Remember, your Pokémon should be strong enough to handle Maylene's Fighting-type Pokémon. You can use Pokémon with strong Flying, Psychic, and Fairy-type attacks to take her down.
Good luck!