The third and last time you fight him is at the Viridian Gym. He is the leader.
The serious last time you beat Giovanni is at the viridian city Pokemon gym.
beat blaine at cinnibar and then Giovanni at viridian city.
you must beat the other 7 gyms
You will encounter team rocket after you beat giovanni, but you will never encounter giovanni in the game again.
You don't 'have' to beat him at all. But beat him as many times as you like, it's up to you.
nothing
Once
You beat a grunt, and when you beat him he drops the LIFT KEY and you can use it to get to the top floor and face Giovanni ;D
You can get a Master Ball at Silph Co. from the Silph Co. President after you beat Giovanni.
Red does not reappear after you beat him.
After you beat Giovanni in Pokémon Red, Blue and Yellow he goes to the gym in Viridian City. He is the final gym leader.
In the 2nd city you went next to the drunk old guy that teaches you. It is in Viridian City by the gym. You have to beat the previous 7 leaders to get here though. Once you beat them, the doors will unlock. Giovanni can also be found on the 11th floor of the Silph Co. building in Saffron City.