In the basement of the Pokemon mansion on cinnaber.
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The Cinnabar gym key is in the abandoned mansion west of the gym. Spray on a super repel and get to work!
You must continue going to the far right of cinnabar while surfing into a cave, there you will have to search and then youl find a key there
You will get the 7th Badge which is the Volcano Badge from Blaine, the Gym Leader of Cinnabar Island however you will need to receive the Key from within the Pokémon Mansion before you can unlock the Gym's locked door.
At cinnabar island at the pokemon lab third door.
Find the secret key in the Pokemon Mansion,( Next to the gym)