This gym specializes in Fighting-type Pokemon. To excel in this gym, you should use Flying, Psychic, and Fairy-type Pokemon. If you don't have those, having Pokemon that knows moves of those mentioned types will help.
The Gym's layout is very straightforward. There's a single corridor where the other trainers are waiting. However, you can't go straight ahead because of the portions on the floor that roll to the opposite direction. You have to step on the switches to light up the side rooms and follow the paths leading back to the main corridor. You have to get past all of the trainers before you reach the Gym Leader Brawly.
Battle Girl Laura (Prize Money: P416)
Black Belt Hideki (Prize Money: P416)
Battle Girl Tessa (Prize Money: P352)
Both of Brawly's Pokemon have Guts as their ability. This ability increases their attacks by 50% whenever they're inflicted by a status condition. Each of his Pokemon has a debuff (Leer, Sand Attack), and a buff (Bulk Up) which can turn the tide of battle if combo'ed with their powerful moves. As long as your defending Pokemon is strong against these attacks, you shouldn't worry. Any other types must mitigate the damage done by using Potions as necessary.
Leader Brawly (Prize Money: P1,920)
After the battle, you'll get the Knuckle Badge. This lets you command Pokemon up to LV30. You'll also receive TM08 Bulk Up. Brawly will also tell you that Steven, the receipient of the Letter you're carrying has head into the Granite Cave north of the town. This is where you're going to go next.