You have to get the other seven Kanto gym badges, and then show those badges to Blue. Blue is on Cinnabar Island, and he will not leave to go to the Viridian City gym unless you show him the seven Kanto badges.
The final gym is the Viridian City Gym. Blue is the Gym leader.
Gym Leader Blue
the 16th gym leader is:Trainer Blue
the last gym leader is Blue.When you go to the seventh gym leader Blaine you should see Blue and asks you if you have 7 badges then you beat Blaine and go to Blue and he says to meet him at his gym
go to the 5 gym
Blue, the last gym leader in kanto region
Blue
The 5th gym is the Fuchia city gym Gym leader Koga
It is Giovanni, the team rocket leader. He's in the Viridian City Gym. Its rumored he murdered the previous gym leader, Blue.
To get the 8th Gym Leader to return to his Gym in Viridian City, you must defeat all 7 other Gym Leaders.
It's Blue
You find him at Cinnabar island.