You can't
Mansion of Hidden Souls happened in 1993.
You will need to use the Secret Key which is hidden in the basement of Pokemon Mansion which is located on Cinnibar Island.
Mansion of Hidden Souls was created on 1993-12-09.
He had to arrange the elements in a specific sequence to unlock the hidden pattern.
In the basement of the Pokemon mansion on cinnaber.
In "Luigi's Mansion," the money collected by Luigi serves multiple purposes. It contributes to the player's overall score and is used to unlock various upgrades, items, and bonuses. Additionally, collecting money enhances the game's replayability, encouraging players to explore the mansion thoroughly to find hidden treasures. Ultimately, it adds to the fun and challenge of the game.
The only way you can unlock it is if you find the key from the mansion on Cinnebar Island
In Final Fantasy VII: Crisis Core, the hidden key in Shinra Mansion can be found in a chest located in the basement area. To access it, you must first solve the puzzle involving the cryptic notes and the order of the paintings in the mansion. The hidden key unlocks a door leading to additional treasures and secrets within the mansion.
The cast of Mansion of Hidden Souls - 1993 includes: Greg Bowman as Jonathan Al Bruce as The Hunter Willis McGruder as Man 2
If you mean, "how do I unlock the Gym where he is", you have to go into the mansion and get to the basement. There you will find the key to open the Gym
The key is in the Pokemon mansion.
The basement.