[This content is out of date and no longer canon]
These aren’t fully set in stone so one or two might change in the future, and the details on just how some of them ended up Gym Leaders is still unknown, but here’s a general outline! (Sorry, the World page section on the Gym Leaders isn’t up to date. I’ll get on that.)
Kanto Gym Leaders
Forrest Nibi (Pewter City, Rock-type Specialist)
→ Brock gives the Gym to Forrest when he decides to pursue pokémon medicine.
Peony Hanada (Cerulean City, Electric-type Specialist)
→ Originally, Misty left the Gym to Daisy when she moved to the Orange Islands, but neither of Daisy’s children were interested in being the next Gym leader when she decided to retire. Violet’s daughter, Peony, on the other hand, loved battling and was the best candidate. She breaks the pattern of the previous Cerulean Gym Leaders by being an electric-type specialist like her father, however.
Adriana Mitler (Vermillion City, Water-type Specialist)
→ With Peony running the Cerulean Gym and Heather up to…. something, Lt. Surge had to look outside his daughters for successors. While I’m still not sure exactly how Adriana ends up running the Gym, she does have a connection to the Lieutenant, being his step-niece.
Erika Tamamushi (Celadon City, Grass-type Specialist)
Kestrel Sekichiku (Fuschia City, Poison-type Specialist)
→ After Janine’s death, Koga comes out of retirement to take over being Gym Leader until his eldest granddaughter comes of age. Kestrel sees being Gym Leader as a chore more than anything and often has to be bribed to be there.
Amaryllis Tokiwa (Saffron City, Fairy-type Specialist)
→ After Sabrina leaves Kanto to pursue acting, her Gym is in…. someone’s hands (not sure yet). Eventually, Amaryllis ends up inheriting it and becomes Kanto’s first fairy-type Gym Leader..
Kaito Mikami (Cinnabar Island, Psychic-type Specialist)
→ Having originally lived on the island many years ago, when Blaine retires, he asks his granddaughter-in-law’s cousin to take over the Gym, much to Kaito’s surprise. He also helps with the rebuilding efforts for the island.
Mimi Aoyama (Viridian City, Multi-type Specialist)
→ When Gary retires to pursue research, it’s no question who will inherit the Gym, since he’d been training Mimi as his successor for a while. She follows in her husband’s footsteps and doesn’t stick to one type of pokémon, using the same team she traveled with on her pokémon journey.
Johto Region
Lyra Suzuki (Violet City, Flying-type Specialist)
→ While the details are still shaky, Lyra inherits the Gym from Falkner when he moves to Kanto due to his marriage with Janine.
Bugsy Hiwada (Azalea Town, Bug-type Specialist)
Whitney Kogane (Goldenrod City, Normal-type Specialist)
Karen Yoshino (Cherrygrove City, Dark-type Specialist)
→ Following Pryce’s retirement, Karen left the Elite Four to pursue her dream of opening a Gym in her hometown.
Yuusuke Hasumi (Ecruteak City, Ghost-type Specialist)
→ He inherits from his father, Morty.
Chuck Sawada (Cianwood City, Fighting-type Specialist)
Jasmine Asagi (Olivine City, Steel-type Specialist)
Draco Fusube (Blackthorn City, Dragon-type Specialist)
→ It is a bit of a tradition that the head of the Mikami-Fusube joint clans is the Blackthorn Gym Leader. The head of clan title switches between the two clans each generation, so while Clair was the previous clan head, Lance’s son Draco is the next one.
Hoenn Region
Rudy Jiang (Rustboro City, Rock-type Specialist)
→ He inherits from his mother, Roxanne. His twin sister inherits their father’s Gym.
Safir Jiang (Dewford Town, Fighting-type Specialist)
→ She inherits from her father, Brawly. Her twin brother inherits their mothers Gym.
Theo Heath (Mauville City, Fairy-type Specialist)
→ Details are still unclear but he inherits when Wattson retires, most likely.
Caleb Moore (Lavaridge Town, Fire-type Specialist)
→ Inherits from his mother, Flannery.
Wally Birch-Fang (Petalburg City, Multi-type Specialist)
→ Norman recommended Wally as his successor, much to Wally’s surprise, since May was busy being a Frontier Brain and Max was more interested in studying under Prof Birch.
Winona Lu (Fortree City, Flying-type Specialist)
Laelia & Trias Jin (Mossdeep City, Psychic-type Specialists)
→ They inherit from their father, Tate.
Lisia Lu (Sootopolis City, Dragon-type Specialist)
→ She changes gears from coordinating to battling and takes over the Gym from her uncle Wallace when he becomes Hoenn Champion.
Sinnoh Region
Roark Subotin (Oreburgh City, Rock-type Specialist)
Gardenia Kobyakova (Eterna City, Grass-type Specialist)
Natalia Rebreanu (Veilstone City, Steel-type Specialist)
→ Maylene leaves the Gym in her hands when she becomes an Elite Four member.
Crasher Wake (Pastoria City, Water-type Specialist)
Ceres Severnaya (Hearthome City, Poison-type Specialist)
→ Fantina befriended Ceres when the girl took an interest in coordinating while she studyed in Hearthome. When Fantina decided to retire to pursue coordinating full time, she named Ceres as her successor. Challengers have to complete math puzzles to reach her.
Maisy Chistyakova (Canalave City, Dark-type Specialist)
→ Byron heard about her through fellow Gym Leader Volkner, as Maisy is his niece. She apprenticed under him and he liked her spunk enough to name her his successor. She’s done…. interesting things to the Canalave Gym decor, in usual Maisy fashion.
Candice Nakamura (Snowpoint City, Ice-type Specialist)
Volkner Tatarintsev (Sunnyshore City, Electric-type Specialist)
Unova Region
Cheren Rysinov (Aspertia City, Normal-type Specialist)
Roxie Helias (Virbank City, Poison-type Specialist)
Gabriele Campanula (Castelia City, Ground-type Specialist)
→ He thought his father’s old job of being a Gym Leader sounded fun, so when Burgh retired to focus on his art, Gabe got himself into the position of next Gym Leader through some dubious methods.
Elesa Tatarintseva (Nimbasa City, Electric-type Specialist)
Stephan Dunstan (Driftveil City, Fighting-type Specialist)
→ Details are still unclear on how he inherits from Clay.
Jerrie Vidales (Mistralton City, Steel-type Specialist)
→ Inherits from her mother, Skyla. The Gym still has an airplane theme to it since Jerrie loves planes so much.
Marlon Naryshkina (Humilau City, Water-type Specialist)
Noah, Micah & Terah Pendragon (Opelucid City, Dragon-type Specialists)
→ As Drayden’s eldest grandchildren, the triplets are the natural choice for his (and Iris’) successors, though Iris’ eldest, Cypress, is gunning for the position as well.
Kalos Region
Viola Leroux (Santalune City, Bug-type Specialist)
Grant Leroux (Cyllage City, Rock-type Specialist)
Chili von Schwan (Shalour City, Ghost-type Specialist)
→ He takes over the Gym when his wife, Korrina, becomes an Elite Four member. His Gym is themed like a magic show, with Chili as the master of illusion and his ghost pokémon as his assistants.
Mairin Sycamore (Coumarine City, Grass-type Specialist)
→ She inherits from her grandfather, Ramos.
Clemont Brassard (Lumiose City, Electric-type Specialist)
Sawyer Sheng (Laverre City, Multi-type Specialist)
→ Details are still unclear but he inherits when Valerie moves to Kanto.
Azariah Arcelus (Anistar City, Dark-type Specialist)
→ I have no idea how he ends up here, but that’s just how it is with Azariah really.
Evelyn de Beauvilliers (Snowbelle City, Water-type Specialist)
→ I’m also still not sure on the details, but she takes over from Wulfric through some series of events.