The Pewter City Gym is the first Pokemon Gym you can challenge in the game. Each Gym in the game has a requirement you must meet before you can challenge the Trainers and Gym Leader. Defeat the Gym Leader to earn a Gym Badge.
The Pewter City Pokemon Gym specializes in Rock-type Pokemon. The requirement for challenging this gym is to have a grass- or water-type Pokemon in your party. Grass-type Pokemon can be found on routes 1 and 2 and the Viridian Forest. Use grass- and water-type attacks to deal massive damage. Ground- and fighting-type attacks will also work against these monsters. The first two Trainers can be challenged back to back or avoided by taking the side paths.
Brock is the Gym Leader of the Pewter City Gym. His Pokemon are both rock- and ground-type, meaning they are 4x weak to water- and grass-type attacks. You can capture an Oddish/Bellsprout or Bulbasaur and eliminate Brock with minimal effort as long as they are around level 10 and have a grass-type attack.
Defeat both of Brock’s Pokemon to receive the Boulder Badge, which allows Pokemon up to level 20 to obey you in battle. You will also receive the TM01 Headbutt, a strong normal-type attack. Leave the Gym to run into a familiar kid. He will introduce himself and hand over 5 Great Balls.
After obtaining the Boulder Badge, a few new items will become available at the Poke Mart. The next step in your adventure is Route 3