You will find Cascarrafa Gym northeast of West Province (Area One).
How to Reach the Gym
To reach the Cascarrafa Gym, go to the southeast part of Cascarrafa and stand on the platform between the two waterwheels to be lifted up to the gym.
Gym Test
When you approach the gym, there will be a cutscene, and you will get Kofu's Wallet. Your Gym Test is to return the wallet to Kofu in Porto Marinada.
To reach Porto Marinada, first you can stand on the platform to go back down to the middle level of the city, then go northwest to the next platform and stand on it to go down to the lowest level of the city. Then keep going northwest until you enter the desert.
Asado Desert
Go west from Cascarrafa to reach Porto Marinada. If you want to battle the Quaking Earth Titan to the west, it is recommended that your Pokémon are at least level 44.
As you go northwest from the desert, there will be a cutscene.
Go northwest to the Pokémon Center and heal up there, then go northwest into Porto Marinada.
Porto Marinada
Make your way down the paved path and you will find a large open air market under a roof. Go west through the market to find Kofu. Approach him and there will be a cutscene.
You will battle Gym Trainer Hugo, who has a level 28 Floatzel and a level 28 Clauncher. You get 3920 Pokédollars for winning.
Auction
To win the auction, you need to place the winning bid without making it cost more than you can afford. You get to keep any remaining money. Feel free to stay in Porto Marinada to try the auctions. You can get different types of Poké Balls, medicines, berries, and more. The Rotom Catalog is a Key Item that you can use on Rotom to change its form.
When you are ready, use the Map App to take the Flying Taxi back to Cascarrafa, and go up to the upper level as before, and enter the gym. Talk to the person at the front desk, and if you are ready to battle Kofu, say Yes.
Water Gym Leader Kofu, the Surging Chef
Kofu has a level 29 Veluza, a level 29 Wugtrio, and a level 30 Crabominable that he Terastallizes, with Tera Type Water. You get 5400 Pokédollars for winning.
You will get a Gym Badge. Check the Gym Badge Effects page to see the level of Pokémon that will listen to you and be easier to catch, or open the Map App or Pokédex App and press X, then select Profile, then press A to see the effects of your current badges.
You also get a TM022 Chilling Water.
New Sandwiches
A new Gym Badge means new sandwich recipes. Be sure to stop by any Every Wich Way and talk to the person in front of the counter to learn some new recipes.