The Gym Leaders of the Pokémon universe are supposed to be the best of the best - local experts with recognized specialization in a specific Pokémon type. They serve as ambassadors for their towns and regions, maintaining their Gyms as crucial proving grounds for serious trainers. But what happens when a random kid rolls through, demolishing their strongest allies with a full roster of six Pokémon that perfectly counters theirs? After conceding defeat and handing over their respective Gym Badges, how do Gym Leaders recover from such a defeat, and what's next for them?
In the first generation of Pokémon games, the first Gym Leader players encounter is Pewter City’s Brock, the Rock-type specialist who presents the first big obstacle in the adventure (if players chose Charmander as their starter, at least). In that crucial, early-game battle, poor Brock only has two Pokémon on his team and is easily handled by any moderately experienced player. It makes sense, then, that Brock gives up the Gym Leader lifestyle in the Pokémon anime to join up with Ash on his journey as a trainer, eventually setting out on an entirely different career path as an aspiring Pokémon breeder.
In the Pokémon video games, however, it appears this doesn't happen often. Most Gym Leaders seem to simply stick around after being defeated, waiting for the next challenger to come through and prove their skill. Between players' visits to Kanto in Pokémon Red and Blue and Pokémon Gold and Silver, the only Gym Leaders that change are Fuchsia City's Koga, who's replaced by his daughter Janine after he's promoted to the Elite Four, and Viridian City's Giovanni, who disappeared at the end of Red and Blue and is replaced by Blue, the first game's rival.
In most games since Pokémon Emerald, players have also been able to rematch Gym Leaders after beating the Champion, showing they maintain their roles after their initial defeats. Pokémon Sword and Shield's Expansion Pass DLC even has Gym Leaders such as Milo and Nessa join the player as a partner in the Galarian Star Tournament, a competition available after finishing the games' main story. These Gym Leader partners provide three of their strongest Pokémon for battle, teaming up with three of the player’s own to form a star-studded team.
As revealed by Brock in Pokémon: Let's Go, Pikachu! and Eevee!, Gym Leaders do have the unique ability to directly challenge the region’s current Champion, meaning they likely have a higher chance of moving up to Champion or Elite Four member than just any random trainer. Perhaps a future Pokémon game will allow its characters to more fluidly change roles throughout the story, maybe even letting the player try their hand at becoming a Gym Leader.