well after that you have technically finished the game to you can do what you want. When I finished it I caught all the legendaries.
there is no 9th gym
You CAN NOT become a Gym Leader in Pokemon HeartGold and SoulSilver or ANY MAIN SERIES POKEMON GAME.
walk in
The gym is in Blackthorn City.The gym leader is Clair.
He has many types.
In Pokémon HeartGold, you can find the 3rd Gym Leader in Goldenrod City.
awww by beating the 7th gym
try beating the gym leader
its in azalea town
mahogany town
Obtain it from a gym leader.
in viridian city