Viridian Gym specializes in Ground-type Pokémon, therefore you should avoid bringing Poison, Rock, Steel, Fire and Electric-type Pokémon which will take in a lot of damage. Psychic-type Pokémon should only be used against Nidoqueen and Nidoking but not for the others since most Psychic-type Pokémon have very bad physical defense, unless they always strike first in battle and is capable of one hit KO their opponents.
Water or Grass-Type attacks can easily take down Giovanni‘s Pokémon, especially Rhyhorn and Rhydon since they have dual-type weaknesses to these attacks. Flying-type Pokémon is immune to Ground-type attacks but the battle may last a little longer. Also, they should be avoided as far as possible against the secondary Rock-type Rhyhorn and Rhydon. Some good Pokémon that can be caught easily and useable in this gym include the starter Squirtle, Gyarados, Poliwag, Krabby and Gastly, due to its high special attack and Levitate ability which makes it immune to Ground-type attacks.
Congrats for earning the Earth Badge from Giovanni, which is the final gym badge of Kanto. With all eight gym badge in your collection, you will be permitted to enter the League Reception Gate in Route 22. After healing your Pokémon in the Pokecenter, go west to Route 22, and further to the end of Route 22 before turning north for the League Reception Gate.
In there, keep walking until you have arrived at a four-path junction. Head to the path north where you will end up in Route 23.