Which city contains the first Gym in Pokémon Red and Blue?

Pewter City contains the first Gym in Pokémon Red and Blue.

The Pewter Gym is led by Brock, a Rock-type specialist. Players normally reach it after traveling through Viridian Forest, making Brock the first Gym Leader encountered in the standard Kanto progression. Defeating him awards the Boulder Badge.

Viridian City also has a Gym, but it is not the first active Gym players challenge in the usual story. Its leader is initially absent, and the location becomes the final Gym challenge after the player has progressed through the other Kanto Gyms. Cerulean City contains the second Gym, led by Misty.

Pewter City also includes the Pewter Museum of Science, an early location that connects the game world to fossils, ancient Pokémon, and the region's wider adventure setting.