You have to get all eight badges.
the gym leader is... Giovanni the head of team rocket.
*SPOLIER* , Giovanni ,
It is the last gym. You should have 7 badges to battle the gym leader. The gym leader is Giovanni.
It is atchrily the 8th gym. It will open when you get the first 7 gym badges
i think she is on route 25
Go to the Pokemon League, West of Virdian City
you find Giovanni in silph co.
the eighth gym is in virdian city, talk to blue on cinnibar island and he will go back to his gym.
cinnabar island. he will go to the gym after getting all the 7 kanto gym badges
You head to Pewter City and battle Brock there gym leader! Then from there you head south to Virdian City. There gym leader won't be there so you keep on to Pallet Town. Then pass Cinnabar island and head to the seaform island and challenge the gym leader! Then after you defeat there gym leader talk to the Virdian Gym leader at Cinnabar island and then you go defeat him at his gym
after you get all 15 badges go to cinnabar island and there will be the 16th gym leader after you talk to him go to the virdian city gym and he will be there
I think you need to beat 7 kanto gym leaders to fight him.