You first need the Devon scope given by steven at the route east of fortree city after getting the Devon scope you can enter the fortree city gym.
the sixth gym is in canalave city
The sixth gym AKA the fortree city gym specializes in Flying Type Pokemon.
The Fortree gym.
The sixth gym is in Canalave City of Pokemon Pearl, west of Jubilife City.
The Sixth Gym leader is Chuck, he has a Hiryama and a Poliwrath.
you dont have to surf to a gym
Sabrina's gym in saffron city
it is in saffron
The sixth gym is in Fuschia City, where you fight Koga, Poison type leader.
sixth gym laeder
The sixth gym is Mossdeep City Gym - Gym Leaders Liza and Tate (Mind Badge). You have to surf right of Lilycove city.
its in pallet town