its actually the 8th gym so you need 7 badges and then it will unlock
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in Viridian city. the one that was locked.
You have to find the key in the abandoned mansion.
The gym in Viridian is actually the last gym. You'll unlock it after getting all the other badges. You should continue to Pewter through Viridian Forest.
I think the gym in fuschia city
In Pokemon FireRed and LeafGreen you can't open a gym.