One island Pokemon centre after you defeat elite four and the 2 rockets at the volcano and find the gem.
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You will find the key to the Gym of Cinnabar Island, From the Pokemon Mansion that is on the same island.
In Cinnabar Island, south of Pallet Town.
You can find the key to unlock the door in Pokemon mansion
Get the 7th badge from cinnibar island then you will see Bill outside of the gym say YES to his question to be transported to one island!
On Cinnabar Island.