The leader is "Blue". Also known as "Gary" in the anime. When you've wn the seven other kanto badges, you go to Cinnabar island, and talk to him. Then go to Viridian gym and battle him
The Viridian Gym leader (Blue) is at Cinnebar Island, you can go there by surfing south from Pallet Town. If you want him to return to his Gym you have to beat the other 7 Gym leaders in Kanto
In the beginning of the game, he isn't there. Towards the end, you will find his rival on cinnabar island by himself. After you talk to the rival, he leaves to go back to viridian. Fight the cinnabar gym leader, go back to viridian and the gym leader should be there. The man you talk to on Cinnabar Island Isn't the gym leader, but his rival. After you speak with his rival on cinnabar, the gym leader reappears in viridian. You have to fight and beat the cinnabar gym leader first.
It is Giovanni, the team rocket leader. He's in the Viridian City Gym. Its rumored he murdered the previous gym leader, Blue.
He's at Cinnabar Island.
Cinnabar Island.
I believe after you beat the viridian city gym leader he disappears.
In the original Silver game, the gym leader was Blue and you would find him at Cinnabar island. After meeting him there, you can go back to the gym and fight him.
The final gym is the Viridian City Gym. Blue is the Gym leader.
The 8th gym leader is in Viridian City.
Gym Leader Blue
Blue
First find the gym leader on cinnibar island then go to viridian city you can now enter the gym.