Kantonian Gym is an unofficial Gym built in Malie City to give Alolan people the chance to experience what challenging a Pokémon Gym feels like in other regions. It is designed after the Vermilion Gym in the Kanto region, and defeating it will earn the challenger the title of "Kantonian". Like the Vermilion Gym, there are multiple trash cans lined up around the Gym, although in here, unlike in the Vermilion Gym, they are purely aesthetic.
Entrance to the Gym costs 1,000, and it can be challenged once per day. The player must defeat the four Gym Trainers around the Gym in order to be allowed to challenge the Leader. Defeating the Leader rewards the player with a Surge Badge, an unofficial Gym Badge that is designed after the Vermilion Gym's Thunder Badge.
After the player has become the Champion of Alola, the Trainers in the Gym start using stronger Pokémon.
After the player has defeated the Gym for the first time, Ryuki will occasionally appear here as well. He will battle the player if talked to, although defeating him isn't necessary in clearing the Gym challenge.
At the top of the Gym tower, Kiawe and Ash fought the Leader of the Kantonian Gym, Ryuki. While Kiawe and Marowak were unsuccessful, Ash was able to defeat Ryuki's Zweilous and Druddigon in a Double Battle with Rowlet and Meltan. In the end, Ash was rewarded with a novelty Badge with Ryuki's face on it.
Unlike in the games, the Kantonian Gym is not themed after any Kanto Gym in particular, with its ninja theming being based on assumptions made on Kantonian culture. As a result, there are a few differences between the Kantonian Gym challenge and a typical Gym challenge in the anime. Unlike most Gyms, visitors must face the challenges of the four Kantonian Gym Trainers before advancing to the Leader. However, these challenges are not Pokémon battles, like in the games, but a ninja-themed set of obstacles.
According to the statues at the entrance, the entry and service fees for the Gym are 5,000 for adult males, 3,000 for adult females, 1,000 for children under the age of 15, and Pokémon enter for free.
As a reference to the game mechanics in previous generations, in the Kantonian Gym, everything is viewed from a bird's-eye view and the Gym Trainers walk up to the player, instead of challenging them from their current positions.
The Kantonian Gym theme is a remix of the Gym theme from the previous Pokémon games.