This is a comprehensive list of the Pokemon Gym Leaders, what Pokemon they use, what Pokemon are best to defeat them, general combat strategies, and what you win. Let's get started.
OVERVIEW: Pokemon Gym Leaders are experienced Pokemon trainers that sport two to five powerful Pokemon. Each trainer specializes in a specific type of Pokemon, so plan your combat team carefully to give yourself the advantage. Pokemon that are NOT of the specialty type will use moves of the specialty type (e.g. Sabrina uses Venomoth, a Bug/Grass, but Venomoth will use Psybeam and Psychic, Psychic types attacks.)
Each city except Pallet Town and Lavender Town has a Gym. Its leader is posted on a sign along with a clue as to what type of Pokemon the leader uses. Each leader is backed by lieutenants whose sole purpose is to prevent you from reaching the leader easily. The lieutenants will often use the same type of Pokemon as the leader.
STRATEGIES
Brock, Pewter City Gym Leader: Specialty in Rock type Pokemon, lieutenants use Ground/Rock types
Ground and Rock types are vulnerable to Water and Grass type attacks such as Squirtle's Bubble and Bulbasaur's Vine Whip. On the other hand, they pretty much shrug off electric attacks such as Pikachu's Thundershock and normal attacks like Pidgey's Gust and Rattata's Tackle. If you don't have Bulbasaur or Squirtle, take your Caterpie and raise it to level 14. It will evolve twice and become a Butterfree with Confusion, a Psychic attack that will cause normal damage to Brock.
Brock uses a level 12 Geodude and a level 14 Onix. Bulbasaur should be at level 13 before facing Brock; Squirtle at level 12; Butterfree at level 14; and Charmander at level 15. When you win, you get the Boulderbadge, which combines with the HM 05 to let you teach and use Flash, and the TM 34 Bide. Bide lets you take damage over the course of two or three turns then pay it back double.
If you have trouble with Brock, remember to stick with your starting Pokemon. Charmander may need to be at level 16 and use Ember despite the fact that it's not very effective. Squirtle at level 11 with Bubble and Bulbasaur at 13 with Vine Whip are your best bets.
Misty, Cerulean City Gym Leader: Specialty in Water type Pokemon
Water types fall before Grass and Electric powers. Bulbasaur (which may have evolved into Ivysaur) or Pikachu are your best bets. Squirtle/Wartortle won't be extremely effective, and Charmander/Charmeleon is at a distinct disadvantage in this battle. If all else fails, use your Pidgeotto (if you have one, which you should.)
Misty uses a level 18 Staryu and a level 21 Starmie. Ivysaur should be at level 19; Pikachu should be at level 20. When you win, you get the Cascadebadge, which combines with the HM 01 to let you teach and use Cut any time. It also lets you control ALL Pokemon up to level 30. You also get the TM 11 Bubblebeam. Bubblebeam is a powerful water attack that is useful early on.
If you can't seem to beat Misty, use either Ivysaur's Vine Whip or Pikachu's Thundershock. You can train your Pokemon west of the city. Charmeleon will get beaten easily, and it will be a long battle with Wartortle.
Lt. Surge, Vermilion City Gym Leader: Specialty in Electric type Pokemon.
Lt. Surge is paranoid (by my standards) and has locked himself behind a security system. Beat the three lieutenants, then leave, heal, and come back. Check the trash cans until you find a switch; the second switch that opens the security doors is in a can adjacent to the first can. If you like, you can play it safe by saving before you try for the second switch. Note that if you don't get the second switch on the first try, the first switch is reset AND it will be in another can, giving you all the more reason to save.
Electric types have the advantage over water types like Wartortle and flying types like Pidgeotto. They don't do so well against Grass types like Ivysaur, but the best type to use is a Rock/Ground type. Geodude (or Graveler, if you have it) fit the bill nicely in this scenario. Fire types like Charmeleon have no advantage or disadvantage against Electric types.
Lt. Surge uses a level 21 Voltorb, a level 18 Pikachu, and a level 24 Raichu. Geodude should be at level 20, Graveler at level 25 (the level at which it evolves,) Wartortle at level 28 (which is too high anyway,) Ivysaur at level 24, and Charmeleon at level 26. When you win, you get the Thunderbadge, which gives you a power boost. You also get the TM 24 Thunderbolt, the ultimate in thunder attacks. Teach it to your Pikachu.
If you're having problems with Surge, remember that ground types are your best bet. If you have Geodude, level it to about 20 and use it. If you don't have Geodude, you aren't in position to catch one easily. Head east and catch Diglett (or, if you REALLY want to beat up on Surge, a Dugtrio) and get Dig (lv. 19), then use that to pound Surge.
Erika, Celadon City Gym Leader: Specialty in Grass type Pokemon.
Erika has a horde of trainers in front of her. When you reach the line of 4 trainers, she is second from the left. Remember that you must talk to her; she will not engage you directly.
Grass types control Rock, Ground, Electric, and Water types, but they are at the mercy of Fire types like Charmeleon. They are also weak against flying types like Pidgeotto.
Erika has a level 29 Victreebel, a level 24 Tangela, and a level 29 Vileplume at her command. Charmeleon should be at level 27, Wartortle at level 29, Pidgeotto at level 28, Graveler at level 29, and Pikachu at level 29 using normal type attacks. When you win, you get the Rainbowbadge which makes your Pokemon unquestionably obey you up to level 50 as well as the TM 21, Mega Drain, which does some MAJOR draining of HP. Useful attack to have.
Should you have trouble with Erika, head west from Celadon and use Cut to get the HM 02 Fly, then teach it to Pidgeotto. Fly is dominant against Erika's Pokemon. If you don't have either Pidgeotto or Charmeleon (shame, shame,) you can catch Vulpix or Growlithe (depending on your version) east of the city. When all else fails, Wartortle and Pikachu should go normal type attacks.
Sabrina, Saffron City Gym Leader: Specialty in Psychic type Pokemon
Sabrina is guarded by a maze of warp tiles. Make it easy on yourself. The correct warp in any given room is the one directly above or below the one you came in from. Sabrina is quite powerful, so you should probably fight all of the trainers preceding her to gain valuable combat experience.
Psychic types have an advantage over Poison types. Bug and Ghost attacks are known to work best against them. However, Gastly, Haunter, and Gengar are Ghost/POISON hybrids and they'll get murdered. Avoid using Ivysaur/Venusaur for this battle since it also has Poison characteristics.
Sabrina controls a level 38 Kadabra, a level 37 Mr. Mime, a level 38 Venomoth, and a level 43 Alakazam. The Pokemon you intend to use for this battle should be around level 46 to do well. When you win, you get the Marshbadge, which makes your Pokemon obey you without question up to level 70. You also get the TM 46, Psywave, the most powerful Psychic attack. Teach it to your Kadabra/Alakazam/Gastly/Haunter.
Sabrina is the "squeeze" stage of the game, so I don't blame you if you're having trouble. Ghost attacks may be super effective against Psychic types, but the three ghosts are all ghost/poison types, and since Psychic is super effective against Poison, you'll get pounded unless you're at level 49-52. Your strongest Pokemon is probably best. If you got Jolteon, its Pin Missile works well. Avoid using Hitmonlee or Hitmonchan.
Koga, Fuschia City Gym Leader: Specialty in Poison type Pokemon
Koga has a troop of ninjas guarding him. He also has a maze of invisible walls. Look closely, though, and they can be seen.
Poison types don't do too well against other Poison types. They also can't get much off of ground types. Your best bet is either a ground type like Graveler, Golem, Sandshrew, Sandslash, Diglett, or Dugtrio, or a Psychic type like Kadabra or Alakazam. Here's a good place to try out that new Psywave of yours.
Koga owns a level 37 Koffing, a level 39 Muk, another level 37 Koffing, and a level 43 Weezing. (Hmm, his Pokemon's levels can do no better than Sabrina? Ouch.) Kadabra or Alakazam can win easily at level 40. Ground type users, it's your call, but I'd recommend level 44. Upon your victory, you get the Soulbadge, which provides a boost in your defensive strength. It also lets you use Surf anytime. You also get the TM 06 Toxic. Toxic is a Poison attack that poisons your enemy in such a fashion that the damage it does increases each turn.
If you have problems with Koga, use a ground type like Golem or Dugtrio. Psychic is super effective as well. You should have some Antidotes and the Poke Flute for this battle to cure poison and sleep.
Blaine, Cinnabar Island Gym Leader: Specialty in Fire type Pokemon
A lot of the doors won't open. You can either talk to the trainers to engage them, or go to the little machines behind them and press A to initiate a trivia game. It's pretty easy to do. If you answer correctly, you won't have to fight. If you're wrong, you'll go into combat.
A good Water Pokemon is your ally in this place, and you should have enough of them to last a loooong while after the trek through the Seafoam Islands. Raise your favorite one and get ready to face Blaine. Fire types also eat it before Rock and Ground types. Omanyte and Kabuto are good choices.
Blaine has a level 42 Growlithe, a level 40 Ponyta, a level 42 Rapidash, and a level 47 Arcanine. Any water Pokemon with a level of about 52 is more than a match for Blaine. When you win, you get the Volcanobadge, which boosts your Special attack powers. You also get the TM 38 Fire Blast, the #1 fire attack. Save it to use on Moltres if you get him.
If you have problems beating Blaine, teach your best Seafoam-caught Pokemon Surf and kick Blaine where it hurts. Staryu and Starmie have the advantage of using Recover to replenish HP which makes them the prime candidates if you don't have Blastoise or Gyarados.
Giovanni, Viridian City Gym Leader: Specialty in Ground/Poison/Rock type Pokemon
Giovanni's gym itself is almost a dungeon. Just navigate it and eventually you'll reach the big guy in the top left corner.
Giovanni will fall easily to a Psychic or Water type. Avoid Fire and Electric powers in this fight.
Giovanni (hey, four sentences starting with Giovanni! Am I good or what?) uses a level 45 Rhyhorn, a level 42 Dugtrio, a level 43 Nidoqueen, a level 45 Nidoking, and a level 50 Rhydon. Crush them like the bugs that they are with a level 55+ Water or Psychic Pokemon. Upon your victory, you get the final badge, the Earthbadge, which makes ALL Pokemon obey you! You also get the TM 27, Fissure, which defeats an opponent in one attack.
Remember, if you're having trouble, Giovanni uses Ground and Poison types. Alakazam will pound the Poison types into the ground and his high Special means that he shouldn't have much trouble with the other Pokemon either. Water or Grass also works well and you should have an abundance of those Pokemon. Venusaur, Victreebel, and Vileplume (is there something about the letter V and Grass types? Hmmm...) are all Grass/Poison hybrids which means they defend well against other Poison types. Avoid Tangela in this combat.
SPECIAL: ELITE FOUR COVERAGE
As a bonus, I have taken the liberty of covering the Elite Four. Here we go! Your best strategy is to have one good Electric type, one good Flying type, one good Water type with at least one good Ice type move, one good Psychic type, one good Fire type, and one good Grass type.
Lorelei, First of the Elite Four: Specializes in Ice type moves
Lorelei's Ice type moves are usually used by her Water Pokemon. Your Electric type should be at level 63 and ready to handle a level 54 Dewgong, a level 53 Cloyster, a level 54 Slowbro, a level 56 Jynx, and a level 56 Lapras. When you win, you get to move on to Bruno.
Bruno, Second of the Elite Four: Specializes in Fighting type Pokemon
Bruno is a Fighting type trainer, but he also has two Onix to round out his squad. Put your Water type in the lead to start the battle, switch to your Flying type for Hitmonlee and Hitmonchan, go back to the Water type for his second Onix, and switch again to your Flying type for Machamp. Your Water and Flying type should be at level 66 and ready to handle a level 53 Onix, a level 55 Hitmonchan, a level 55 Hitmonlee, a level 56 Onix, and a level 58 Machamp. When you win, you get to face Agatha.
Agatha, Third of the Elite Four: Specializes in Ghost and Poison type Pokemon
Agatha trains Poison types, which means that a good Psychic type is your friend in this battle. It should be at level 68 and it should be ready to face a level 56 Gengar, a level 56 Golbat, a level 55 Haunter, a level 58 Arbok, and a level 60 Gengar. Upon your victory, you can move on to Lance the Dragon Trainer.
Lance, Leader of the Elite Four: Specializes in Dragon type Pokemon
Dragon types fall before Ice moves, but they can defend well against Fire, Water, Grass, and Electric powers. If your Water type knows an Ice move (which it should, you were warned), that's the one to use. Your water type should be at least level 66 and should be ready to face a level 58 Gyarados, a level 56 Dragonair, another level 56 Dragonair, a level 60 Aerodactyl, and a level 62 Dragonite. When all is said and done, he proclaims you the Pokemon League Champion...
Or, he would have, but your rival got there first. You must best him in combat if you are to win. This will be the greatest test of your preparedness. Good luck.
YOUR RIVAL: THE MOMENT YOU'VE TRAINED FOR
Your rival uses a balance of Pokemon. Be ready to go. Your Water type should be in the lead. Hopefully, he has at least 2 PP left in his Ice attack. If not, Water attacks will suffice.
Your rival (I will call him Gary) will always lead off with a level 61 Pidgeot (use Ice attacks), a level 59 Alakazam (use your strongest Pokemon), and a level 61 Rhydon (use Water attacks). The second half of his team is determined by the Pokemon he chose at the beginning of the game.
If Gary chose Bulbasaur, you will fight a level 61 Gyarados (use Electric powers), a level 63 Arcanine (use Water attacks), and a level 65 Venusaur (use Flying or Ice attacks.)
If Gary chose Squirtle, you will fight a level 61 Arcanine (use Water attacks), a level 63 Exeggutor (use Ice or Flying attacks, but refrain from Psychic attacks), and a level 65 Blastoise (use Electric attacks.)
If Gary chose Charizard, you will fight a level 61 Exeggutor (use Ice or Flying, but not Psychic), a level 63 Gyarados (use Electric attacks), and a level 65 Charizard (use Electric or Ice attacks.)
WHEW! When you win, you get to see the end. Your six Pokemon are put in the Hall of Fame. All that's left is... CATCH EM ALL!
Good luck!