A Gym is a place where Trainers go to strengthen and train their Pokémon through Pokémon battles against publicly recognized Trainers known as a Gym Leader. In all regions that include gyms, there are 8 Gyms, each with a different type specialty which are apart of and the preliminary battles which allow a Trainer to take part in a Pokémon League.
Unlike the main series, Gyms in this series work differently. Trainers must match the amount of Pokémon the Gym Leader has and in each round, each Trainer selects one of their Pokémon to face off against the other. When a Pokémon faints, the round is over and whichever persons Pokémon remains is the winner of that round. The winner of the Gym Battle is whoever ends the match with the most wins. In the case of a tie (when a Gym Leader has two, four, or six Pokémon where a tie is possible), the Trainer must face-off against one of the Gym Leaders surviving Pokémon using one of their own suriviving Pokémon, and the winner is determined through that. If the Trainer wins, they are presented with a Gym Badge.
A Gym Leader (Japanese: ジムリーダーGym Leader) is the highest-ranking member of a Pokémon Gym. The main job of a Gym Leader is to test Trainers and their Pokémon so that they are strong and resourceful enough to compete against the Pokémon League.
A Gym Leader's job is not just to test upcoming Trainers. They must oversee how their respective Gym is run and to maintain it. Great care must be taken to make sure that their Gym reflects the best environment for the Pokémon type and playing style of choice, usually an environment that suits that type of Pokémon used there (like pools in a Water-type Gym or flowers and plants in a Grass-type Gym). According to Elesa, teaching young people how to battle with Pokémon can also be the job of a Gym Leader.
It has not exactly been made clear how Gym Leaders attain their position, but it appears that it varies for each Gym. Some of them founded their Gym and are self-appointed; others are chosen by the other members of the region's Pokémon League. A few appear to be passed on through families: Nessa and Flannery all take over their respective Gyms from a close family member. However, Gyms do not necessarily belong to a particular family; Marlon and his sister Nessa are both Gym Leaders at the same time, in different Gyms. Leaders can also choose to vacate their position.
Brock is the Pewter Gym Leader specialising in Rock-types. He ran the Gym from the age of 11; he ia also a keen breeder. He has often temporarily left the Gym under the leadership of his younger brother, Forest.
Misty is the Cerulean Gym Leader specialising in Water-types. She is a professional athlete in swimming.
Lt. Surge is the Vermillion Gym Leader specialising in Electric-types. He is a former and now retired army officer.
Erika is the Celadon Gym Leader specialising in Grass-types. She is a florist and gardener, its also been said Erika secretly cultivates the designs for all of Kanto's greenery sites.
Janine is the Fushia Gym Leader specialising in Poison-types. She took the Gym over from her father, Koga when he became a member of the Elite Four.
Sabrina is the Saffron Gym Leader specialising in Psychic-types. Originally, she had to fight for her Gym against the Fighting-type Dojo next door, however they were no match for Sabrina's psychic powers. She is said to receive lovecalls from the International Police to help with cases, of which she declines.
Blaine is the Cinnabar Gym Leader specialising in Fire-types. He is a failed researcher and instead wishes to become a TV star, specifically a game show host, albeit unsuccessfully.
Gary is the Viridin Gym Leader who doesn't specialise in one type. He took the Gym over after Giovanni abandoned it and Gary was beaten as Champion, shortly after he took a trip away with Lexi.
Milo is the Turfield Gym Leader specialising in Grass-types. He is a farmer and professional sheep herder.
Nessa is the Hulnbury Gym Leader specialising in Water-types. She is an aspiring model, and is good friends with Elesa.
Kabu is the Motostoke Gym Leader specialising in Fire-types. He is a former Boxing Champion and Coach.
Bea is a Stow-on-Side Gym Leader specialising in Fighting-types. She is an aspiring kick-boxer who wants to compete in the olympics.
Allister is a Stow-on-Side Gym Leader specialising in Ghost-types. He was scouted while out on an essential trip for more energy drinks, and was too scared to say no.
Opal is the former Ballonlea Gym Leader specialising in Fairy-types. She is a former fashion model, who retired and took on Bede as an apprentice.
Bede is the Ballonlea Gym Leader specialising in Fairy-types. He originally specialised in Psychic-type, until Opal took him on as her apprentice and successor. He was disqualified from the Gym Challenge due to destorying the Stow-on-Side mural.
Gordie is a Circhester Gym Leader specialising in Rock-types. He truthfully only got a position as a Gym Leader due to his mothers nature for sleeping with senior members of the board - she strongly denies this was with Chairman Rose, however.
Melony is a Circhester Gym Leader specialising in Ice-types. She is said to have got her Gym Leader title due to questionable actions.
Piers is the former Spikemuth Gym Leader specialising in Dark-types.
Marnie is the Spikemuth Gym Leader specialising in Dark-types.
Raihan is the Hammerlocke Gym Leader specialising in Dragon-types.