Gym Leader

Gym Leaders are the head Pokemon Trainers in any given gym in all of the Pokemon video games. The names of the Gym Leaders and types of Pokemon they specialize in are varied depending on which version of Pokemon you're dealing with. Some are as follows (they are in the format Type of Pokemon: Name of Gym Leader (pokemon they use if available)):

Blue/Red Versions...

Rock Type: Brock Water Type: Misty Electric Type: Lt. Surge Grass Type: Erika Psychic Type: Sabrina Poison Type: Koga Fire Type: Blaine Final Gym: Giovanni

Gold/Silver Versions...

Flying Type: Falkner Bug Type: Bugsy Normal Type: Whitney Ghost Type: Morty Fighting Type: Chuck Electric Type: Jasmine Ice Type: Pryce Dragon Type: Clair

Ruby/Sapphire Versions...

Rock Type: Roxanne Fighting Type: Brawly Electric Type: Wattson Fire Type: Flannery Normal Type: Norman Air Type: Winona Psychic Type: Liza and Tate Water Type: Wallace

Emerald Version...

Rock Type: Roxanne Fighting Type: Brawly Electric Type: Wattson Fire Type: Flannery Normal Type: Norman Air Type: Winona Psychic Type: Liza and Tate Water Type: Wallace

Diamond/Pearl Versions...

Rock Type: Roark (Geodude, Onix, Cranidos) Grass Type: Gardenia (Cherubi, Turtwig, Roserade) Fighting Type: Maylene (Meditite, Machoke, Lucario) Water Type: Crasher Wake (Gyrados, Quagsire, Floatzel) Ghost Type: Fantina (Drifblim, Gengar, Mismagius) Steel Type: Byron (Bronzor, Steelix, Bastiodon) Ice Type: Candice (Snover, Sneasel, Abomasnow, Medicham) Electric Type: Volkner (Raichu, Octillery, Luxray, Ambipom)

As shown above, depending on the version, the Gym Leaders and types they specialize in vary.

TIPS:

In the Diamond/Pearl version, I thought I'd let you know - Fantina, the Ghost-Type user, is VERY irritating to beat (for me, at least, I chose Piplup at the starting). So, when you battle her, only use moves that will inflict damage. If you need to heal your best Pokemon, then send out a weak one, and let it heal your strong ones so that your strong ones don't take damage while you heal. She may take a few tries to beat.