There are two prerequisite events that have to be completed before you can access the Silph Co. building on "Pokémon Leafgreen." You'll need to defeat Team Rocket's leader Giovanni in his secret base in Celadon City, and then you'll need to rescue Mr. Fuji from the top of Pokémon Tower.
- Defeat Giovanni
The entrance to Team Rocket's base is hidden in the arcade in Celadon City. Defeat the conspicuous Team Rocket member, then check the poster behind him Giovanni is found at the end of the base and gives up the Silph Scope when defeated.
- Get to the top of Pokémon Tower
With the Silph Scope, you can now identify and battle the ghostly Pokémon inside of Pokémon Tower in Lavender Town. You can also identify and fight the Marowak that blocks access to the top floor. After beating some Team Rocket grunts on the top floor, you'll find Mr. Fuji, who gives you the Poke Flute.
- Enter Silph Co.
Once you have the Poke Flute, the guard who was stationed in front of Silph Co. disappears, allowing you to walk right into the building. Once it is completed, Team Rocket retreats from Saffron City, and previously inaccessible buildings, like the Gym, open.