How do you unlock viridian city gym?

1. Defeat Brock, the Gym Leader of Pewter City.

2. Defeat Misty, the Gym Leader of Cerulean City.

3. Defeat Erika, the Gym Leader of Celadon City.

4. Defeat Lt. Surge, the Gym Leader of Vermilion City.

5. Defeat Sabrina, the Gym Leader of Saffron City.