Pokémon FireRed and LeafGreen/Route 10

This place is East of Cerulean City, and this route also seems to be the entrance area of Rock Tunnel. Going through Rock Tunnel may seem like a roundabout route, but it's the only way, because there is a Snorlax blocking Route 12 and the thirsty officer blocking Saffron City will not let the player in.

Coming from Route 9
Going to Route 9 is necessary to go to Route 10, but it can only be accessed by having a Pokémon that knows Cut.

Catch A Gyarados
Gyarados can be found in Route 12, once you find the Super Rod. The Super Rod will have to wait until later, but it won't be too long until you go South of Route 12 into the house where the Fishing Guru's younger brother lives. But that will only be after Rock Tunnel and Pokémon Tower are finished, and the Poké Flute is obtained.