first you have to beat the seven gyms, then talk to blue on cinnibar and he will go back to his gym which you cn now beat
The last Kanto gym is in Varidian City.
you can take on any gym leader in kanto in any order (excluding blue you have to battle him last)
blue is the last gym leader in kanto after you beat the elite 4 you can go to kanto by taking the ss aqua in olivine city and go to kanto, battle the gym leaders, and capture legendaries
You have to battle the last gym leader Blue in viridian city in kanto reigion.
After you beat the other 7 kanto leaders you will find Blue in the rubble of Cinnebar Island. Talk to him and he will return to Viridian Gym, then you can go battle him.
To get the last gym badge you have to go to cinnabar island and you will meet blue. he will tell you that he will be waiting in veridian city to battle you. when you go to veridian city you go to the gym and fight him at the end of the gym.
In what game exactly?
you have to beat all the kanto gyms then go to cinnabar and talk to blue he will then go to the last gym and you can battle him there
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.
After defeating Koga, who is the fifth gym leader in the Kanto Region, just move on to the next city. You can get the gym badges out of order, so just move on to a city that you haven't defeated yet. Save Cinnabar Island for last, though, because it definitely is the toughest gym.
In the first city
You need to do the last gym in kanto.