vermillion city
In the first city
actually there isn't 9 gym leader there is only 8 like the other ones. Actually, the 9th gym leader in heartgold and soulsilver would be the first gym leader in Kanto after you beat the elite four the first time, you can face the kanto gym leaders in any order.
blue in viridian city
you have to go to funchisa city in kanto
you can take on any gym leader in kanto in any order (excluding blue you have to battle him last)
Pewter City
He is in the cave on Seafoam Island, which is east from Cinnabar.
Red is in mt.silver and blue is the eight kanto gym leader
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.
Clair and she is the leader of blackthorn city or if you mean the kanto leader its blue/Gary ashes rivle
You have to battle the last gym leader Blue in viridian city in kanto reigion.
ok he is no longer the gym leader. his daughter took over the gym. he is now the elite four trainer. i think the second one.