It is not, actually. It was a remake of G/S/C.
>Much like how Pokémon FireRed and LeafGreen revisited the Generation I story of Kanto, HeartGold and SoulSilver retell the story of Johto, with the player's starting town being New Bark Town. -Bulbepedia
I guess they put different gym leaders to make it more interesting.
Hope This Helped :D