First of all, you must have a Pokemon that knows strength. then move the boulders into the stairs they will fall down.. move all the boulders into the stairs and make them fall then you will easily get to the gym leader.
and if you want to defeat him easily use an ice type Pokemon that knows ice moves like ice beam. its super effective against dragons.
or you can use a Pokemon that knows mirror move because dragons are weak against their own moves.
NOTE : Make sure you have lots of potions and high level pokemons like 50+ to make sure you can win.
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.
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.
The eighth gym leader still awaits, as well as the whole of Kanto.