After getting rid of Team Aqua/Magma in the Weather Institute and battling your neighbor in Route 119, you reach Fortree City.

Explore Town

In the house next to the Pokémon Center, the girl there will give you a Skitty if you give her a Pikachu.

There is a Desk and Chair Shop where you can get decorations for your Secret Base.

There is an old woman who will give you TM10 Hidden Power if you give the correct answers to her questions.

If you try to enter the Gym, you discover that something invisible is blocking the way.

Catch Kecleon

Make sure to get some Ultra Balls and Great Balls from the Poké Mart, then go east to exit Fortree City. You will be in Route 120. You will come back here again later, but for now, walk as far east as you can go, then go southwest through the tall grass to find a bridge. Walk across the bridge and you will find Steven. Talk to him. He will use the Devon Scope to make an invisible pokémon turn visible.

It's a wild Kecleon. It's rare in the wild, so try to catch it!

After you catch or defeat the Kecleon, Steven gives you the Devon Scope, allowing you to make other invisible Kecleon appear.

Go back to Fortree City and try to go to the Gym again. There is still something invisible blocking the path. Check on it and say that you want to use the Devon Scope. The invisible Kecleon appears and then runs away.

Now you can go into the Fortree Gym.