u can find it in Pokemon mansion.
If you are talking about the gym on the island you go into the old building and when you reach the bottom you should find a key of game shark cheats
you must find the key to the cinnabar island gym......
Explore the ruined mansion to find the key.
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
Fly to pallet town and go south eventually you will reach cinnibar island on the island is a mansion go in and find the secret key and when you do unlock the gym and go in and beat the 7th gym leader.
u have to find the key in the Pokemon mansion i think
To get in blaine's gym ,you have to go into the big building next to it and find your way through it. Far into the building you will find a key. Use the key on the gym door and it opens. Hope this helps:)
I believe it is on Cinnabar Island. To enter the gym, you have to get through the Pokemon Mansion and find the secret key.
The only way you can unlock it is if you find the key from the mansion on Cinnebar Island
Get the key from the basement of the mansion next to the gym.
its inside the mansion
You have to go to the old mansion beside the gym and get a key