You have to defeat Team Rocket at the Radio Tower in Goldenrod. To get in you need a team rocket costume, you can get it in the underground where the photographer has his stand. After that you have defeated team Rocket you can move east from Mahogany to the Ice cave.
you have to beat the last gym leader
you beat the last gym leader in johnto
You beat the 6th gym leader.
beat the 8th gym
You beat Chuck, the 4th gym leader, in Cianwood city. After you beat him, leave the gym. His wife will come up to you and give the HM to you.
beat gym leader
Simply beat the gym leader in every gym.
get stronger then you are before
you have to beat the last gym leader
you beat the last gym leader in johnto
you go west of ecturek and face the gym leader of olivine
You beat the 6th gym leader.
Beat the gym the gym leader will request you going in there but you will have to fight
level up your Pokemon and get many full heals
beat the 8th gym
He's called Green and he's the gym Leader of the last Gym. Beat all the other gyms then beat him.
You CAN NOT become a Gym Leader in Pokemon HeartGold and SoulSilver or ANY MAIN SERIES POKEMON GAME.