A Magmar level 54, a Macargo level 54, and a Rapidash level 59.
You CAN NOT become a Gym Leader in Pokemon HeartGold and SoulSilver or ANY MAIN SERIES POKEMON GAME.
Not enough info: which gym leader?
It's near the Seafoam Islands.
In the goldenrod gym.
the 15th gym leader in soul sliver is blaine and he uses only fire type pokemon
beat gym leader
Brock.
Go east of Cinnabar Island to the Seafoam Islands.
defeat gym leader.
in seafoam island
Jeff Gordon
He is the Gym Leader in Cinawood.