You must get the Devon Scope from Steven on Route 120, then you return and press 'A' when your reach the invisible barrier blocking your way to the gym. (Its actually a Keckleon in stealth)
The Fortree gym.
Fortree City.in fortree city
At fortree city.
the gym is in fortree city and it is flying type gym
winona is the flying type gym leader
The 6th gym is located in Fortree City.
you goto the fortree gym.
Fortree City
fortree
fortree
its........................... cecleone!
Fortree City.