You need to go through the Pokemon mansion and beat all the trainers. At the last one you get the "secret key" to Blaine's gym.
Sabrina's gym in saffron city
Step on the portals until you get to the gym leader.
The sixth gym badge (the Marsh badge) is located at Saffron City's gym. It is in Gym Leader Sabrina's possession, so you have to defeat her to obtain it.
In the Kanto region, the sixth gym is in Saffron City. The gym leader is Sabrina.
the sixth gym is in canalave city
The sixth gym leader,Sabrina, is in Saffron City.
The key to the gym
The sixth gym AKA the fortree city gym specializes in Flying Type Pokemon.
the 5th gym is saffron gym with sabrina!
The Fortree gym.
The fifth Gym Leader in Pokemon LeafGreen is Koga, the leader of Fuschia City's Gym. He specializes in Poison-type Pokemon and hands out the Soul Badge.
The sixth gym is in Canalave City of Pokemon Pearl, west of Jubilife City.