You first need to beat Giovanni at Silph co which is in saffron city once you do that you can go into the gym and receive your sixth badge from Sabrina.
leaf
Saffron city.
You recieve the sixth badge after defeating Sabrina in the Saffron City gym.
Saffron city.
Fight the seventh gym on Cinnabar Island. Then fight the eighth gym at Virirdian City.
it is in saffron
its in pallet town
The sixth badgeno actually it the third badge
Fortree City ( Feather Badge ) Leader's pokemon: Altaria, Swellow, Skarmory, Pelipper
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.
the sixth gym badge is the jet badge and the type they have are flying type hope this helps:]
in gym durr