From Steven at Fortree city. The thigh blocking you from going into the gym is an invisible Kecleon.
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all you really have to do is go up to an invisible Pokemon (keckleon, kekleon? forgot the spelling of it) and once your in front of it, press "A". The Pokemon will appear and either battle you or flee.
You can't catch wild Treeko in Pokemon Emerald. You need to choose it out of the three Starter Pokemon that are offered to you at the beginning of the game: Treeko, Torchic, and Mudkip. to see it, battle rival or other trainer
you need to be in a dark spot.
its not an emerald its RAYQUAZA the main legendary Pokemon in Pokemon emerald.
See www.gameshark.com or supercheats.com