You will find the key to the Gym of Cinnabar Island, From the Pokemon Mansion that is on the same island.
7th floor.
You can find the key to unlock the door in Pokemon mansion
He will appear after you beat the 7th gym leader.
As soon as you get the 7th badge and the HM rock smash.
It is in Viridian City. After you have beten the 7th gym leader it is open
to open the door of the 7th gym!
Get the 7th or 8th gym badge, and its in the Power Plant.
Use the HM surf and go south from pallet town you will eventually reach cinnibar island the gym is locked though to open it find the key in Pokemon mansion.
The 7th gymbadge is in Cinnabar Island
Arcanine at level 48.
7th floor.
you have to go into the pokemon mansion on cinnabar island and navigate your way around there untill you find the key
there is a key in the Pokemon mansion... you need it to get into the gym... search the Pokemon mansion thoroughly
You can find the key to unlock the door in Pokemon mansion
He will appear after you beat the 7th gym leader.
seventh badge is cinnabar island. Fly to Pallet Town, surf south. Then go into pokemon mansion and find the secret key to unlock the gym
Its inside a pokeball inside the Pokemon mansion on Cinnabar Island