Like all other Gyms in Paldea, the Cascarrafa Gym building itself is simply used for registering Gym challengers. The Gym Test and Gym battle are both conducted outside the building, with the Gym Test taking place in Porto Marinada.
Once the player reaches the Cascarrafa Gym, Kofu will run out of the building in a hurry, headed to the Porto Marinada auction, but without his wallet. The Gym staffer will ask the player to deliver Kofu's Wallet to him at Porto Marinada.
The Gym Test begins after the player reaches Kofu at Porto Marinada and defeats his Gym Trainer. Kofu will give the player 50,000 to make the winning bid for the legendary wakame seaweed from Hoenn; the player keeps whatever amount is not spent. After receiving the seaweed, Kofu will return to Cascarrafa to battle the player.