You just have to beat all the other 7 gyms, then he should be back.
team rockets boss { Giovanni }
The gym leader is Giovanni, leader of Team Rocket. You have to come back to Virdian last as he is the Eighth gym leader. THE LAST ONE!
The Viridan City Gym leader is the leader of Team Rocket Giovanni after you defeat Team Rocket one last time, go to gym he will be waiting for your battle and then you get your badge!
you will have to beat all the gymleaders
You have to go Cinnabar Island. He'll be standing around the place with three rock climbs outside the gym. You have to have beat all seven gym leaders before him. Talk to him and he'll go back to the gym.
beat all the other gyms first and silph co etc.
You fid him back in his gym in veridian city. The place where Giovanni use to be
Giovanni is only avaliable for his gym battle after you have gained the other 7 gym badges. Once you have done that the Gym will unlock in Viridian for your final match.
don't you just go back to Viridian and defeat Giovanni
The leader which is Blue (Gary) is in Cinnabar Island but won't go back to Viridian until you get the other 7 Kanto Badges. The gym is in Seafoam Islands, which is East (left) of Cinnabar Island. You go up the ladder as soon as you enter.
by training it in pewter gym, route 3, viridan forest, and mt.moon 2
Giovanni is the final gym leader.