OK, basically, I'm gonna tell you the best way to get past the Gyms in Yellow.

Pewter Gym (Brock): Right away, the game puts you at a disadvantage- every Pokémon in Pewter Gym is Ground type, and you're forced to start with an Electric-type. Yes, I know it's technically a Rock Gym, so why don't you tell that to the Jr. Trainer with a Diglett and a Sandshrew, hmm? He doesn't seem to have gotten the memo. Anyways, it's not him you should be worried about, it's Brock. Defeating him with Pikachu, while not impossible, is nevertheless rather difficult, and you want the easy way. Thankfully, the programmers were compassionate enough to give you a few options you didn't have in Red and Blue- namely, Nidoran and Mankey. On Route 22, west of Viridian City, you can catch Mankey and both varieties of Nidoran. Mankey learns Low Kick at level 9 (couldn't even get the move in Red and Blue), and both Nidoran get Double Kick at level 12 (couldn't learn it until much later in the game in Red and Blue). Both moves are Fighting, and give you the type advantage over Brock's Rock-types. Another option is to catch a Caterpie and raise it to Butterfree. Once Metapod evolves into Butterfree at level 10, it learns Confusion. While it's not super effective against Rock-types, it does target their weaker Special stat, and it's a far better option than anything Pikachu can do.

Cerulean Gym (Misty): Pikachu will fare much better here, as Thunder Shock is super effective against Misty's Staryu and Starmie. Still, though, Bubble Beam hits pretty hard, so I would recommend getting the Bulbasaur from Melanie right there in town. If Bulbasaur's not your style, Routes 24 and 25 are home to wild Bellsprout and Oddish, both of which are just as good at getting the job done. Bellsprout is my personal recommendation, Victreebel being my favorite Grass-type, but any of the three will do well. Just make sure you raise them to a high enough level before the battle!

Vermillion Gym (Lt. Surge): As much fun as I find it to take on his Raichu with only Pikachu, like Ash did, there's an easier way. In Diglett's Cave, which is just outside of town, you can, as the name would imply, find wild Diglett. However, on rare occasions, Dugtrio will appear, and will likely be higher in level than anything else you got on hand. Bring a Flying-type with you, battle a wild Dugtrio with said Flying-type (because it's immune to Dugtrio's Dig, which is very powerful), and catch it. You now have something that can take out anything in Vermillion Gym with one attack.

Celadon Gym (Erika): A lot of you probably have Charmeleon by this point, and while it'd definitely do well in here, the sad fact remains that as far as Fire attacks go, it's stuck with Ember for a long time, so you may want some backup. Fortunately, between Celadon City and Cycling Road, you can catch wild Doduo and Fearow in the tall grass, and you don't even need to have battled Snorlax to get to it! Catch one of those, and teach it Fly from the HM you get not far from there, train it up some, and Erika shouldn't be too tricky to defeat. Oh, and if you have any Water-types you've been training, you can get a TM for Ice Beam from a girl on the roof of Celadon Dept. Store in exchange for a beverage (I forget which- just give her one of each, she gives useful TMs in any case). Any Water-type not named Magikarp can learn Ice Beam and instantly overcome its weakness to Grass.

Fuchsia Gym (Koga): In this game, all of Koga's Pokémon are Bug/Poison- three Venonat and a Venomoth. Remember my advice for beating Erika? It also works here, except for that part about Ice Beam. Also, if you still have that Dugtrio from when you battled Lt. Surge (provided you took my advice), you can get a TM for Rock Slide from the girl on the roof of Celadon Dept. Store in exchange for a beverage.

Saffron Gym (Sabrina): This is where things get tricky- Psychic-types in this game are pretty much king. They're only weak to Bug moves, and the best Bug move in this game is Twineedle. Yeah. Fortunately, it's not all bad- Sabrina's team in this is the entire Abra line, known for high Special and Speed, and horrendous Defense. You want something that can hit hard and fast with physical attacks. Like Dugtrio, actually- it's one of the fastest Pokémon in the game, and any Dugtrio you catch in Diglett's Cave already comes with Dig, which in this game has the same power as Earthquake. If you've been training Dugtrio, you can wreck Sabrina's team with ease. Warning- Alakazam might still be faster, but it could also try to use Recover on the first turn.

Cinnabar Gym (Blaine): By this point, you really shouldn't need help. You had to use Surf to get here, so if you taught it to something you've been training well at all, Blaine's Fire-types are toast. Good Water-types for this purpose include Gyarados and Tentacruel. Additionally, this is yet another location where Dugtrio comes in handy- Fire-types are weak to Ground attacks like Dig.

Viridian Gym (Giovanni): Water-types will take you far here, as will whatever Grass-type you used against Misty if you bothered to keep raising it. And of course, Dugtrio will have its moments to shine. First we have Giovanni's own Dugtrio to contend with- as you probably know by now, its Speed is virtually unmatched. Worse, it has Fissure, a move that KOs the target in one hit, provided it hits. Granted, its accuracy is crap, but it has been known to hit. It won't work on faster opponents or Flying-types, so my recommendation is to use a Gyarados- immune to Ground moves, and a Surf or two from it will mark the end for his Dugtrio. Nidoking and Nidoqueen can be taken out in similar ways, though beware of Nidoking's Thunder attack. Alternatively, as Poison/Ground-types, they're weak to Ground themselves, so Dugtrio comes in handy there. Persian isn't a Ground-type at all, and thus will kinda throw a Mankey wrench into your plans. It's fast, but not too difficult to take out- I personally took it out using Pikachu, though if you got something with a halfway decent Fighting move, that'll work great. Unfortunately, decent Fighting moves are hard to come by. Last one is Rhydon, and for his strongest Pokémon, it's an absolute joke. Have any decently leveled Pokémon use any Water or Grass move against it for an instant KO- it has a quadruple weakness to both types, so it takes four times the damage it'd ordinarily take from them, and it also has a low Special, so it'd be taking a lot of damage from them even without the weaknesses.

And there you have it- if you want to beat the Gym Leaders with ease, this is how you go about it. I hope this guide was a big help to you all.

Great job with this man! I can tell that you put a ton of effort into it. This is extremely helpful, and I enjoyed reading it. You added some humor, which was nice. You also included tons of information, some of which I didn't even know. For example, I never knew how OP Dugtrio is in this game. This is a really great gym leader guide. Oh, and nice title by the way

First thought were whoa. This is going to take a while and I know this person took their time with this which is good. Lots of info and stuff I never did. Great job. Dungtrio is a sexy beast in this game which I never knew. I did the exact same thing as you recommened in pewter Gym and Celladon but I would have never though of using Dugtio ever in my life.