From Bulbapedia, the community-driven Pokémon encyclopedia.
Gringey City (Japanese: グンジョウシティ Gunjou City) is a location that exists only in the Pokémon anime. It is located in between Celadon City and Fuchsia City.
According to a map that Misty owned, Gringey City was originally bustling with activity, but the city eventually over-expanded the factories and power plants, resulting in the town becoming virtually abandoned as well as the sea nearby becoming polluted. It was featured in the episode Sparks Fly for Magnemite. Ash's Pikachu was sick, later determined to be caused by a problem with buildup of electricity in the creature's cheeks, and the nearest Pokémon Center was located there. An uncharacteristically ill-tempered Nurse Joy greets Ash.
During their time the power in the town goes out. This was due to a large amount of Grimer clogging the tunnels that deliver water to power the hydroelectric generators. Ash, Brock, and Misty go to investigate the power plant and restore normal operations.
Pikachu, along with the Magnemite and several Magneton were able to ward off the Grimer. They also weakened a Muk, which Ash ended up capturing. Power was restored and the waterways were cleared. The residents left in the city say that they will work hard to clean it up and make it beautiful again.
It is likely that Gringey City is based on the Kanto Power Plant in the games, since Grimer and Muk were added to the Power Plant in Pokémon Yellow.
Pokémon seen in Gringey City