The only way you can unlock it is if you find the key from the mansion on Cinnebar Island
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The 7th gym leader in fire red is Blaine in Cinnabar Island.To unlock The gym, u must go 2 the pokemon mansion,search the gym key in the basement.
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.
It serves one purpose and that is to unlock the Cinnibar Island gym.
I believe it is on Cinnabar Island. To enter the gym, you have to get through the Pokemon Mansion and find the secret key.
You can find the key to unlock the door in Pokemon mansion