The first Gym Leader you will face on the road to collecting all eight Kanto Gym Badges is Brock. He is the Pewter City Gym Leader, specializing in Rock Type Pokemon. Because Brock is faced so early in your journey, his Pokemon Team won't be all that difficult to defeat regardless of the Starter Pokemon you selected. However, if you did pick Bulbasaur or Squirtle as your Starter Pokemon, you will have the clear advantage over Brock since he is especially weak against both Grass Types and Water Types.
If you happened to choose Charmander as your Starter Pokemon, you will probably need to add an additional Pokemon on your way to Pewter City to better help you with Brock's Geodude and Onix. The best Pokemon for this job is Mankey, which can be found on Route 22 just west of Viridian City. In addition to being weak against Grass Types and Water Types, Brock's Rock Type Pokemon are also weak against Fighting Types like Mankey.
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