Once you've beaten Giovanni for the last time, anotherwords completed all the Team Rocket "invasion" missions. Sorry to be a spoiler.
you must beat all other gym leaders
No one, there are only 9 gym leaders
To get the 8th Gym Leader to return to his Gym in Viridian City, you must defeat all 7 other Gym Leaders.
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.
To get to the 8th gym, you needed to have met Blue on Cinnabar Island, you will find him looking at the volcano there. You must have defeated the other 7 gym leaders before he will consider battling you. He will return to his gym, allowing you to face him.
Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
Blue (The eighth gym leader) is waiting on Cinnabar. He won't battle you unless you've beat the rest of the gym leaders
defeat all the other gym leaders
Grass and Electric.
You best the rest of the Kanto Gym Leaders. Since they are challenged in no particular order, Misty may be your Seventh challenge. If this is so, then you go to Cinnibar Island, where you find Blue (Gary Oak's game counterpart). He is the Viridian Gym Leader, and you take your 8th gym challenge. If Misty isn't your 7th challenge, then you beat the other gym leaders before you can challenge Blue.
There is an inn where 3 Gym Leaders battle you (on the Battle Zone)
To find blue defeat all of the gym leaders and then go to cinnabar island blue will be there and will accept your challenge if you have defeated all of the gym leaders. Then go to viridian city gym and get through all of the trainers. After this you can also battle red.