Janine I think
You CAN NOT become a Gym Leader in Pokemon HeartGold and SoulSilver or ANY MAIN SERIES POKEMON GAME.
Not enough info: which gym leader?
In the goldenrod gym.
beat gym leader
Brock.
Kanto's eighth Gym Leader in Heartgold/Soulsilver is a Trainer named Blue, who lives in Pallet Town and is Professor Oak's grandson. He is a versatile Pokemon user, meaning that he uses multiple types instead of other Gym Leaders, who specialize in one type.
defeat gym leader.
in seafoam island
Jeff Gordon
He is the Gym Leader in Cinawood.
in blackthorn city
Simply beat the gym leader in every gym.