Enter Fortree Gym without the Devon Scope glitch is a sub-glitch of Surf on land.
1) Go to this spot in Route 119
2) With an Acro Bike, do a sideways hop upwards and aim to hold A immediately when you land. The exact time is 15 frames counting Brendan's shadow as the first frame. This may take many tries.
3) Surf and go north to Fortree City. Be careful not to get off by going off a cliff or the wrong bridge. Luckily, entering a battle 'won't' mess up the glitch and you can surf through tall grass.
4) Go on the bridge south of Fortree Gym then go off by pressing up where only one tree is directly north of you. You'll be able to access the gym.
As an invisible Kecleon will still block the player, be warned that you'll need a Pokémon with Teleport to escape, or you could get the Gym Badge and then use Fly.
- ChickasaurusGL (article text)