Although I would probably prefer to have a mixed variety like Gary Oak(aka Blue) in Gold/Silver etc., if I had to have one specific type, I would probably choose Grass.

The team I would have would most likely consist of Sceptile, Victreebel, Bellossom, and Celebi. 

I would use the Dark type because, believe it or not, that type has never been used. I would use Poochyena, Mighthyena, Houndour, Houndoom, Murkrow, and Honchkrow. My gym would be the last gym in a region, and give out the Dusk Badge.






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I would probably have a steel type gym because they have pretty good defense and can deal major damage.

I would have dark type gym because there are no dark type gym. There are elite 4 that use dark type but no gym. That why iI choose dark type.

If I had a Pokémon gym it would probably be a dragon type gym because they have no weakness except for ice but I would probably think of a way to stop any  ice type users with a dragon Pokémon with two types so it wouldn't be affected or I would probably just teach it a super effective move like fire punch so my gym would be unbeatable.

Ice with fire combination . and my badge will be called Shear badge. this sentences is not included i will now end this comment or any thing else you call this

Probably Dragon because:
1. They only have 2 weaknesses
2.They normally have very high stats when fully evolved
3.Most of their moves are quite powerful

NUZLOCKED649 : I would have a water type gym with 5 luvdisks and a magikarp...actually for blastoise

I would, no doubt, have a psychic gym because that is my favorite Pokémon type. 
My   Team*:
Espeon
 Gardevoir
  Slowking
 Darmanitan
  Xatu
Mime Jr.
*The Pokémon below are just my favorite psychic types (not counting legendary Pokémon), I'm not sure the exact Pokémon I'd pick for my gym

My Gym would be the 5th gym and it would be fire gym. This would be my team
Team:
Charmeleon lvl 35 - Flamethrower - MetalClaw - Dig - FireFang      Held item: Charcoal
Camerupt lvl 34 - LavaPlume - Bulldoze - RockSlide - Rest         Held Item: Chesto Berry
Arcanine lvl 34 - FireFang - ThunderFang - ExtremeSpeed - Roar      Held Item: Charcoal
Infernape lvl 37 - FocusBlast - FlameWheel - Acrobatics - Dig       Held Item: Sitrus Berry

Rematch Battle Team:
Charizard lvl 70 - AirSlash - Flamethrower - DragonPulse - FlareBlitz         Held Item: Charcoal
Camerupt lvl 70 - Earthquake - Flamethrower - StoneEdge - Fissure        Held Item: LeftOvers
Arcanine lvl 70 - Crunch - ExtremeSpeed - Flamethrower - WildCharge     Held Item: Flame Plate
Chandelure lvl 70 - ShadowBall - Will-O-Wisp - FlameBurst - Psychic       Held Item: Ghost Gem
Magmortar lvl 70 - Flamethrower - SunnyDay - SolarBeam - HyperBeam      Held Item: HeatRock
Infernape lvl 73 - Flamethrower  - Earthquake - CloseCombat - FlareBlitz     Held Item: LightBall

And my badge would be called "The Torch Badge"

Did I post in here already...?

My gym would be a Steel gym. Skarmory, Aggron, Steelix, Scizor, and Magnezone would be my team.

I would surely be the strongest gym leader ever. I would focus on various types so I would be a Pokémon champion instead. I do love dragons which is my main team being:
Salamance
Dragonite
Charizard
Altaria
Raquaza
Gyrados

My gym would be the last gym in the region, then I can have a ryperior, gigalith and even a tyranitar!!!

I would use Grass type Pokémon if I were a gym leader.

I really like using Grass types.

Nah screw dragon i'd go with meh bugs






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I would be a water and fire type gym because those are in the 4 best types and my fav

Since my favorite type is the dark type... my Gym would have the dark type.

My team:

Umbreon (Female), Starla -- Shadow Ball, Dig, Faint Attack, and Toxic
Murkrow (Female), Meina -- Drill Peck, Confuse Ray, Shadow Ball, and Faint Attack
Houndoom (Male), Raetym -- Thunder Fang, Flamethrower, Shadow Ball, and Iron Tail
Sableye (Male), Jirend -- Shadow Ball, Power Gem, Aerial Ace, Poison Jab
Scrafty (Male), Deiamin -- Crunch, Dragon Claw, Sludge Bomb, Drain Punch
Zweilous (Female), Nyreina -- Dragon Rush, Crunch, Ice Fang, Aqua Tail


The reward for defeating my Gym? The Shadow Ball TM and a Random Pokemon Egg from the below list... the numbers next to it determine the rarity. 10 is the most common; 1 is rarest. 

Eevee (1)
Murkrow (7)
Sneasel (4)
Houndour (6)
Larvitar (3)
Poochyena (10)
Seedot (9)
Sableye (4)
Carvanha (7)
Cacnea (6)
Krabby (7)
Absol (3)
Stunky (10)
Spiritomb (3)
Skorupi (8)
Purrloin (6)
Sandile (8)
Scraggy (7)
Zorua (1)
Pawniard (5)
Vullaby (6)
Deino (3)

And that's it! Well, and the bad. But I haven't figured out a name for it yet.