You need to have defeat the other gym leaders of Kanto first, after that you will find Blue on Cinnabar Island, talk to him and he will return to his gym.
defeat all the other gym leaders
beat the other 7 gyms in kanto and talk to the 8th gym leader on cinnabar island. he will leave and go back to the gym
Blackthorn city, in kanto you find him at cinnibar island show him your 7 kanto badges to make him go back to the gym then you can battle him.
It is Giovanni, the team rocket leader. He's in the Viridian City Gym. Its rumored he murdered the previous gym leader, Blue.
you have to use the HM surf and surf behind the gym and go in the cave. go to the other side of the dragon temple and go in. awnser the old man's questions and the gym leader will come in and give you the badge. This is to get the 8th Johto gym badge. To get the Kanto one search: how do you get to the 8th Kanto gym badge
Giovanni Is The 8th Gym Leader In Kanto, And He Is The Leader Of The Evil Organisation, Team Rocket.
Somehow. Go to Cinnabar Island after getting all other badges, talk to him, and he'll go back to the gym.
Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
If you are talking about the 8th gym leader in Johto, you go to New Bark Town, surf east, and go to the Pokemon League. If it's the 8th gym leader in Kanto, go to Saffron City to receive a Hoenn starter from Steven Stone.
Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
You go south of pallet town there should be a little island where cinnabar island was. The 8th gym leader should be there but he won't battle you.You need the 7th gym badge to battle him.The 7th gym is near the island.Beat the 7th gym leader.Talk to the 8th gym leader again and he's going to go back to the gym to what for you there in Viridian City.
The 8th gym leader is currently away from his gym at viridian city, and is at Cinnabar town, and will only challenge you once you have 7 kanto badges. You will have to surf there, and he will be the only person there on the island. Once you talk to him, he will go back to his gym. Hope this helps!