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.
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its actually the 8th gym so you need 7 badges and then it will unlock
You have to find the key in the abandoned mansion.
celio is on the first island you can go there after you win at the cinabar gym, bill will be wating for you outside the gym
I think the gym in fuschia city
You beat the TRAINERS first then the doors unlock and you can go through them ^^