How to get to the 5th gym

Petalburg City, unlike the previous city, has a gym. You won't be able to battle here, though, since this is supposed to be the fifth gym, and you need to visit the first four gyms to get the first four badges before coming here. After you heal your Pokemon and go to the Mart, you'll want to go on to the next city. But you can't, because a guy stops you from going on to Route 104.

Because of this, you'll have to go into the gym, and talk to the gym leader. He's also your father. A person called Wally comes in, and tells your dad he wants to catch a Pokemon. Your dad gives him a Zigzagoon and a PokeBall, and tells you to make sure that he catches a Pokemon. Now just sit back and enjoy the catching. He'll take you back to the gym, and you have to talk to the gym leader again. After that, leave the gym. Done with that? Go west to the next route. In Emerald, before you can leave, a mysterious guy walks up to you and tells you that he's in search for good trainers.