The third gym leader is Whitney.
In the goldenrod gym.
Go to the third
The final gym is the Viridian City Gym. Blue is the Gym leader.
Not enough info: which gym leader?
Gym Leader Blue
You CAN NOT become a Gym Leader in Pokemon HeartGold and SoulSilver or ANY MAIN SERIES POKEMON GAME.
You don't. Go south and you will be able to move it after the third gym leader.
beat gym leader
in blackthorn city.
Brock.
After you defeat the gym leader in Goldenrod City, go next door, to the flower shop, and pick up the watering can. After that, head north toward the National Park.
The eighth gym leader still awaits, as well as the whole of Kanto.