cinnabar island. he will go to the gym after getting all the 7 kanto gym badges
Its in virdian city. The gym leader is Govanni, and he uses ground and fighting pokemon.
Pewter City
I think you need to beat 7 kanto gym leaders to fight him.
*SPOLIER* , Giovanni ,
the gym leader is... Giovanni the head of team rocket.
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
nowhere
go north to the viridian forest and that will get you out.
You can get Pollywhirl in Pokemon Leaf Green in Virdian City.
Its to the LEFT of Virdian City
you find Giovanni in silph co.
Go to the Pokemon League, West of Virdian City