Pokemon - http://maxpages.com/jonezpokemon/Gym

Name: Takeshi (Japanese), Brock (English)

Gym Location: Nibi City (Japanese), Pewter City (English)

Badge: Boulder

Element: Rock, Ground

Pokémon: Geodude and
Onix

Strategies:

Brock's Pokémon are both Rock type, so be sure not to use Pikachu or Charmander. Use a Water or Plant type to defeat him, and you should have no trouble at all. Geodude is very easy to defeat, but Onix is a little harder since his Defense is so high; save your toughest Pokémon for him and make sure their HP is very high throughout the battle, especially when Onix uses BIDE.

Pokémon over level 10 would be a good choice to use in this battle.

Name: Kasumi (Japanese), Misty (English)

Gym Location: Hanada City (Japanese), Cerulean City (English)

Badge: Cascade Badge

Element: Water

Pokémon: Staryu and Starmie

Strategies:

Misty only has two Pokémon, but they're powerful to a beginning trainer. Do not use any Ground, Fire, Rock and Flying Pokémon in this battle and use elemental attacks with Plant and Electric Pokémon. Pikachu's thundershocks and thunderwaves will do great damage. Staryu shouldn't too hard of a battle, but lookout for Starmie's Bubblebeam attack, it can kill even electrical Pokémon. Misty may use a XDEFEND item on her Pokémon during the battle.

Pokémon over level 18 would be a good choice to use in this battle.

Lieutenant Surge; leader of the Vermilion City Gym

Name: Matisse (Japanese),
Lt. Surge (English)

Gym Location: Kuchiba City (Japanese), Vermilion City (English)

Badge: Thunder Badge

Element: Electricity

Pokémon: Voltorb, Pikachu, and Raichu

Strategies:

Lt. Surge uses only electrical Pokémon, so your Ground and Rock Pokémon would be a good choice to use in this battle. If your Rock or Ground Pokémon know DIG, use it! Voltorb can be defeated easily, and the Pikachu can be defeated easily too, but lookout for Raichu; its Thunderbolt attack can defeat your Pokemon if their levels are too low. Buy some Paralysis Cures/Para-Heals because you may need to use them in this battle since many of the electrical attacks can shock your Pokémon into paralysis.

Pokémon over level 24 would be a good choice to use in this battle.

Name: Erika

Gym Location: Tamamushi City (Japanese), Celadon City (English)

Badge: Rainbow Badge

Element: Plant

Pokémon:
Victreebel,
Tangela and
Vileplume

Strategies:

Erika doesn't look mean, but don't underestimate her Pokémon. The other trainers in her Gym should give you a good idea what to expect, so if you can't beat them easily, build your Pokémon's levels up and come back some other time. Do not use your Water Pokémon in this battle; if you chose Charmander at the beginning of the game, use him now. See! Charmanders are a good choice! It should be pretty easy to beat Victreebel and Vileplume, but lookout for Tangela's Bind attack; it's a lot like Ekans' and Arbok's Wrap, and also Bellsprout's Bind attacks which can make your Pokémon defenseless. It might be a good idea to buy some Awakening(or bring the PokéFlute), Paralysis Heal and other items that cure attacks from Erika's Pokémon.

Pokémon over level 28 would be a good choice to use in this battle.

Name: Natsume (Japanese), Sabrina (English)

Gym Location: Yamabuki City (Japanese), Saffron City (English)

Badge: Marsh Badge

Element: Psychic

Pokémon: Kadabra, Mr. Mime, Venomoth and Alakazam

Strategies:

If you captured a Ghost Pokémon in the Lavender Tower, use them now!! Sabrina's Pokémon can be very tough, especially when they use Psybeam, which can take down your Pokemon in one hit! Her Pokémon also use attacks like Confusion, so make sure you don't fight with a confused Pokemon unless it is a Fire type, and then use Ember. Be sure to use elemental attacks like Thundershock and Fire Spin, and bring lots of Hyper Potion!

Pokémon over level 35 would be a good choice to use in this battle.

Name: Kyou (Japanese), Koga (English)

Gym Location: Sekichiku City (Japanese), Fuchsia City (English)

Badge: Soul Badge

Element: Poison
Pokémon: Koffing, Muk, Koffing, Weezing

Strategies:

Koga's Poison-type Pokémon can easily be beaten with Ground and Psychic Pokémon, so the TM that Sabrina gave you should help a lot. The Dig ability is also a big help, since the Poison attacks can't hit you. Bring a lot of Antidotes and Full Heals. Lookout for Weezing's Toxic attack. It takes a lot of HP out of your Pokémon throughout the battle.

Pokémon over level 35 would be a good choice to use in this battle.

Blaine; Leader of the Cinnabar Island Gym

Name: Katsura (Japanese), Blaine (English)

Gym Location: Gren Town (Japanese), Cinnabar Island (English)

Badge: Volcano Badge

Element: FIRE!

Pokémon: GROWLITHE, Ponyta, Rapidash, and ARCANINE!

Strategies:

Fire!! Water Pokémon will be a big help against Blaine, especially since all of his Pokémon are the Fire type. If you captured Articuno, its Blizzard, and Ice Beam will do the trick. Like in Sabrina's gym, judge yourself on the levels of the other trainers in the Gym; if you can't beat them, then go train your Pokémon some more. Bring some Burn Heals and or Full Heals to prevent your Pokémon from fainting from the burn damage the Growlithe and Arcanine will do! Remember, they are especially powerful......

Pokémon over level 40 would be a good choice to use in this battle.

Giovanni; leader of the Veridian City Gym

Name: Sakaki (Japanese), Giovanni (English)

Name: Tokiwa City (Japanese), Viridian City (English)

Badge: Earth Badge

Element: Ground/Poison/Normal
Pokémon: Rhyhorn, Dugtrio, Nidoqueen and Rhydon

Strategies:

Even though you've defeated him before, Giovanni's Pokémon are stronger than last time; Water, Grass and Ice type should help in this battle. Lookout for Nidoqueen and Nidoking, they are dual-element Pokémon. Also lookout for Dugtrio's Dig attack, it can cause a lot of damage to a Pokémon who are at a disadvantage. You'll want to stock up on Hyper Potions, and plan your strategy wisely. A Flareon with Fire Spin will do wonders against the leader of Team Rocket.
Pokémon over level 45 would be a good choice to use in this battle.