Click below to see what each Gym Leader has when they battle you and what you get for beating them

I am going in order of when you fight them

Back to the Labyrinth
Brock: Pewter City
Misty: Cerulean City
Lt. Surge: Vermilion City
Erika: Celadon City
Sabrina: Saffron City
Koga: Fushia City
Blaine: Cinnabar Island
Giovanni: Viridian City