You have to find the key in the abandoned mansion.
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its actually the 8th gym so you need 7 badges and then it will unlock
You don't until you beat the 7 other gyms. After that fly back to Viridian city and the leader will have returned
the 6th gym is saffron city - enter saffron by obtaining tea from the old lady in celadon and offering it to the guard in the house on route 7. open the gym by defeating team rocket/giovanni in silph co.
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.
The 7th gym leader in Pokemon yellow is in Cinnebar Island, which is south of Pallet Town. You need a Pokemon with Surf to get there.