Sabrina's gym is located in Saffron City, but it's inaccessible in Yellow. The entire Saffron City is inaccessible in Yellow because it's the location of the Silph Co., which is the main setting of the game.
In Yellow Version, you need to focus on capturing Pokémon from Kanto and participating in the events that take place in the Silph Co. The entire story revolves around the kidnapping of a famous Pokémon researcher, and you need to save him and deal with the villainous Team Rocket.
However, there is no gym in Saffron City in Yellow Version, so you won't be able to battle Sabrina.