Gym 6 - Gym 7

Vermillion City/ Camp Z - Gym 6
Defeat Mme Z, head East to Diglett's Tunnel

Diglett's Tunnel
Flooded, surf West to Pewter City

Pewter City - Gym 7
Defeat Brock, go South through Viridian Forest

Viridian Forest
Scarier than before! Keep going South to Viridian City...