The Dewford Gym leader, Brawly, specializes in Fighting-type pokémon. Defeat him to earn the second Gym Badge and use Flash outside of battle.
The trainers here use Fighting pokémon. They are weak to Flying, Ghost, and Psychic moves. You should avoid bringing Normal, Rock, and Dark pokémon.
The gym is dark except for a small spotlight on you. The spotlight gets bigger with each trainer you defeat. Inside of this gym, the trainers all have Meditite and Machop. The Machops are weak to Flying and Psychic moves, while the Meditites are weak to Flying and Ghost moves. In Ruby and Sapphire, the trainers' pokémon are level 12 and 14, while in Emerald they are all level 13.
The path around this gym is to go north from the entrance, then east to battle a trainer. After that, if you are playing Emerald, you can go northeast to fight two trainers. Otherwise, go north, then west to fight another trainer. Then in Ruby and Sapphire, go north and then west to fight another trainer. Otherwise, in Emerald, go northwest to find two more trainers. In Ruby and Sapphire, Brawly is in the northeast corner of the gym. In Emerald, he is in the northwest of the gym.
In Ruby and Sapphire, Brawly has a level 17 Machop and a level 18 Makuhita. Both are weak to Flying and Psychic moves.
In Emerald, Brawly has a level 16 Machop, a level 16 Meditite, and a level 19 Makuhita. The Medidite is weak to Flying and Ghost, while the other two are weak to Flying and Psychic.
After you defeat Brawly, you get the Knuckle Badge, which lets you use Flash outside of battle. He also gives you TM08 Bulk Up, which increases Attack and Defense.
Your next destination is Granite Cave, which is northwest of Dewford Town. Along the way, you will pass a couple of Fisherman in Route 106.