To get past the invisible Kecleon blocking the Fortree Gym, you need the Devon Scopes. The Devon Scopes are obtained on Route 120 atop the Northern Bridge by talking to Steven. Steven will ask to observe how you battle, and if you tell him you're ready to battle, you'll engage in a battle with a wild level 30 Kecleon. Now beat the Kecleon (or flee from it, either works), and Steven will give you the Devon Scopes.
Source: Experience
