For the benefit of those new to the Pokémon experience, Gyms are places where Pokémon trainers gather to fight. There are eight Gyms in the Kanto region and each Gym specialises in a specific Type. In the case of the Pewter City Gym, it’s Rock-type Pokémon. Challengers must fight their way through the Gym trainers, before going up against the Gym Leader.
As you enter the Gym, you’ll be stopped by a man wearing glasses. He’ll inform you that you cannot enter unless you know your Type Match-Ups. To prove your knowledge, you’ll need to show him a Water or Grass-type Pokémon that’s super-effective against Rock-types. The easiest choices are Oddish/Bellsprout or Bulbasaur if you’re lucky.
Assuming you have an applicable Pokémon, proceed north to challenge the two Gym trainers. Although if you’re feeling especially confident and/or you’re rushing through the game for some reason, you can avoid both of the Gym trainers by going behind them. The choice is yours, but we’ll document both anyway.
Looking at their Levels, the trainers here are a small step up from the weaklings you fought in Viridian Forest. Anyway, Geodude is exceptionally weak to Grass and Water-types. Unless you’ve traded, you’ll only have Grass-types though, so use those if possible.
This is literally the same Pokémon used by the previous trainer. We suggest using this opportunity to train your Grass-type. That is, unless you’ve been power-leveling your partner or something.