you jump off into the black and you go down the stairs
In the basement of the Pokemon mansion on cinnaber.
go to the Pokemon mansion and go left and there is a door. go in the door and talk to the guy in the purple suit.hes the guy who owns the mansion.
There's a key. You need to go into the mansion on the island, it's in a room in the basement that's difficult to get to... after you get it you can open the door to the gym.
NO!
GO to the Pokemon mansion in cinnabar island and press the statues until you reach he basement and u will find a key and it is used to open the gym's door My name is Kakooli and thank u for reading the answer and thanks
The key is in the Pokemon mansion.
You can find the key to unlock the door in Pokemon mansion
That key unlocks the door to the cinnibar island gym.
you have to work your way through the Pokemon mansion and go out the right door while using the correct switches its and the very end of the Pokemon mansion
Go to the Celadon Pokemon Center and go around behind it. Keep going through that little path and you will reach the backside of the pokemon mansion. You will notice a door, so you'll need to go through it to get into the pokemon mansion
behind the door is leaf green u enter the Pokemon leaf green world
You need the secret key found in Pokemon mansion.