Hey! I suggest not trying to take out this gym if you are new to the game or donot have lvl 100s. These gyms have been taken over by higher levelled trainers and therefore have higher levelled pokemon in them.
The gyms you need to defeat to control Pokemon is in generation 3. This is an NPC gym so it works differently than the normal gym. The first gym is located in Springdale (look for the basic badge gym sign)
There is a thread explaining what pokemons are inside each gym badge here: http://forums.pokefind.co/index.php?threads/quests-challenges.11210/ look « gym badges » guide. Also to be able to start the gyms badges look 1st the « other quest guide ». You need to complete the « visions of victory » quest to open gym badges if I’m not wrong.