Here it is: the first Pokémon Gym of the game! That’s right, it’s the Pewter City Pokémon Gym, and you’re about to fight a few Rock-type Pokémon. Rock-type Pokémon are resistant to a few types that you probably have a fair amount of by now: Normal, Flying and Poison, just to name a few resistances.
You’ll need to target their weaknesses in order to break through those defenses. Grass- and Fighting-type moves are the two types you’ll have without trading at this point in the game to combat the Rock-type Pokémon of this Gym.
In order to enter the Gym in the first place, you’re going to need to show the guide at the entrance a Grass- or Water-type Pokémon. Since Water-type Pokémon are not available at this point in the game without trading, your options are pretty much limited to showing him either a Bellsprout or a Bulbasaur, unless you’ve done some trades, of course. Bellsprout are very common right outside of Pewter City on Route 2 while Bulbasaur are rather rare inside of Viridian Forest.
Either of these two Pokémon will make short work of the Gym, although Pikachu, Eevee, Nidoran♂, and Nidoran♀ all get Double Kick, a Fighting-type move that will do a reasonable amount of damage against the Rock-type Pokémon you’re about to fight. However, since you need to have a Grass-type Pokémon in order to enter the Gym, unless you’re looking to challenge yourself here, you’re really best just using Bellsprout or Bulbasaur’s Vine Whip in here. Make sure you reorder your Grass-type to the lead position in the top-left by pressing R/L while in the Party menu or selecting the Move Pokémon option.
Due to how the Gym is arranged, you can actually skip both of the Trainers in the Gym if you want to simply by walking around them. What’s the fun in that, though?
Both Trainers should be no problem as long as you are using a super-effective move. If you’re struggling with either of them, go catch a Bellsprout at a reasonable level on Route 2 and then return, because it should be a piece of cake with that.
Anyway, with the two Trainers out of the way, the only one left in the Gym for you to fight is the Gym Leader: Brock!
Geodude shouldn’t be much of a problem, although its Tackle can do a decent amount of damage to Pokémon with lower defense. Your partner Pikachu or Eevee won’t take too much from it and can also do a lot with Double Kick.
Onix is surprisingly fast and can also take physical hits pretty well, so Vine Whip probably won’t be enough to knock it out in one hit. Headbutt may also cause your Pokémon to flinch and miss a turn, so be careful of that, and Bind will do damage over time while also preventing your Pokémon from switching out.
After beating Brock you’ll be rewarded with the Boulder Badge, the first of eight Gym Badges. With that, traded Pokémon up to level 20 will obey you and won’t goof off, which is otherwise a risk of traded Pokémon. Every Gym Badge you has increases that limit by 10, and once you have all eight, traded Pokémon will never disobey.
You’ll also be rewarded with TM01 Headbutt, the first TM — Technical Machine — of the game. TMs are multiple-use items that can teach your Pokémon the move contained within them as often as you’d like. TM moves are often pretty powerful, and Headbutt is a move you will definitely want to teach as many of your Pokémon as possible at this point in the game. If any of your Pokémon have Tackle or Scratch, you can replace it with Headbutt without giving it a second thought.
After the whole dialogue with Brock is over, you can shake your Joy-Con or Poké Ball Plus to trigger a cutscene with your Pikachu or Eevee boasting about its win in the Gym. Talking to whichever Pokémon you have following you will also result in some special attention.