Like past Pokémon games, there are 8 gyms which you need to get badges in. However this time, the type they use is not necessarily their Pokémon type, but the type of attacks that their Pokémon tend to use. Below are the 8 gyms and their rostas:

#1 - Roark - Oreburgh City

The first gym leader, Roark, of Oreburgh City is a Rock Type Trainer. His Pokémon are all Rock Type and use Rock Type attacks. Defeat him and you get the Coal Badge and TM76. You can also use Rock Smash outside of battle

Geodude Onix Cranidos
Lv. 12 Lv. 12 Lv. 14

#2 - Gardenia - Eterna City

The second gym leader, Gardenia of Eterna City focuses on the Grass Type Pokémon and attacks. In this gym you have to find the 4 girls hidden around the gym and battle them before Gardenia shows herself. For defeating her you get TM80 and the Forest Badge. You can also use Cut outside of battle

Cherubi Turtwig Roserade
Lv. 19 Lv. 19 Lv. 22

#3 - Maylene - Veilstone City

The third gym leader is Maylene of Veilstone City. Focusing on Fighting Types, you have to do a puzzle that clears the path to Maylene, battling other trainers on the way. For defeating her, you get the Cobal Badge. You can also now use Fly outside of battle

Meditite Machoke Lucario
Lv. 27 Lv. 27 Lv. 30

#4 - Crasher Wake - Pastoria City

The fourth gym leader is Crasher Wake of Pastoria City. Crasher Wake and his trainers focus on Water types and as such so does the puzzle. Requiring you to alter the water levels of the gym so you can get to Crasher Wake, you have to fight all the trainers on your way there. He gives you the Fen Badge. You can also use Fog Clear outside of battle

Gyarados Quagsire Floatzel
Lv. 27 Lv. 27 Lv. 30

#5 - Fantina - Hearthome City

The fifth Gym Leader is Fantina in Hearthome City. First having to find her by the Contest Hall, Fantina is a Ghost Pokémon Trainer. Her Gym is on various levels, once you take the lift up you have to pick a room. 2 out of the 3 rooms contain trainers. The other allows you to go up. After 4 of these, you find Fantina. She gives you the Relic Badge. You can also use Surf outside of battle

Drifblim Gengar Mismagius
Lv. 32 Lv. 34 Lv. 36

#6 - Byron - Canalave City

The sixth Gym leader, Byron requires you to traverse the many levels of the gym using platforms to go up a level or around, battling trainers along the way. The Canalave City Gym is a Steel Type Gym. Byron gives you the Mine Badge for defeating him. You can also use Strength outside of battle

Bronzor Steelix Bastiodon
Lv. 36 Lv. 36 Lv. 39

#7 - Candice - Snowpoint City

The penultimate Gym Leader is Candice of Snowpoint City. This gym is full of Ice Types and as such the Puzzle requires you to make your way to Candice by sliding down the ice taking Snowballs out of your way so that you are able to get through in a Straight Line to Candice. She gives you the Icicle Badge. You can also use Rock Climb outside of battle

Snover Sneasel Medicham Abomasnow
Lv. 38 Lv. 38 Lv. 40 Lv. 42

#8 - Volkner - Sunyshore City

The final Gym Leader is Volkner. Being an electric trainer, his gym requires you to create a path to him by pressing buttons on gears that rotate the bridges secured on them so that you can get to the end. He gives you the Beacon Badge. You can also use Waterfall outside of battle.

Raichu Ambipom Octillery Luxray
Lv. 46 Lv. 47 Lv. 48 Lv. 49

Now with all 8 Badges, which you can look at and use as a sounding board (they make noise when you tap them), you can now head to Victory Road and then on to the Elite Four