You can battle all of the gym leaders out of order in the Kanto Region. Thus, the perspective of the 15th gym leader will change from person to person. Chronologically, Blaine is the 15th gym leader, and he is in the Seafoam Islands, and is up the first ladder you see when you enter.
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Go to Blackthorn city. The gym is there
its brock
go to the next gym leader.
The third Gym Leader in Pokémon HeartGold who is Whitney the Goldenrod City Gym Leader, she'll be in the Radio Tower when you arrive in the city. You have to get the Radio Card for the PokéGear before you can battle her.
The Mahogany City gym leader is not in Cianwood. He is in the gym in Mahogany, not anywhere else.