The eighth gym is back in Viridian City, where you have already been, but it won't unlock until you have all of the rest of the badges. Once you defeat Blaine at the Cinnabar Gym, you can take Route 21 back to Pallet town and either continue to Viridian, or fly there. The gym leader is someone you have met before.
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Find the secret key in the Pokemon Mansion,( Next to the gym)
You can find the key to unlock the door in Pokemon mansion
You can obtain the 8th gym badge (the earth badge) in fire red from Giovanni in Viridian City.
you capture the ledgon dariy Pokemon
You have to go into Pokemon mansion. Find the key in the basement.