It is atchrily the 8th gym. It will open when you get the first 7 gym badges
Its in virdian city. The gym leader is Govanni, and he uses ground and fighting pokemon.
Go to the Pokemon League, West of Virdian City
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
i think she is on route 25
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
*SPOLIER* , Giovanni ,
the gym leader is... Giovanni the head of team rocket.
go south from pallet town and go to cinibar island there will be the 8th kanto gym leader but you need 7 gym badges to get him back to virdian city so you go to seafoam islands(east from cinibar in the sea) and go up the ladder then you will fight blaine he has fire type Pokemon then go back to cinibar and show blue(8th gym leader)then go back to virdian city and challenge blue
It is the last gym. You should have 7 badges to battle the gym leader. The gym leader is Giovanni.
you find Giovanni in silph co.
beat the gym