I would have a Fighting Type Gym. My badge would be the Titan Badge.
The gym would be located in a city and use an entire city block. The battlefield would be outdoors with high rocky terrain on either side of the field, and a row of trees and shrubbery buffering the rocks from the initial battlefield. The battlefield itself would consist of wide starting platforms on either side resting above a deep pool, and in between the starting platforms would reach several smaller rectangular platforms (not floating, just sticking out above the water) with surrounding circular platforms (these are floating). I would also point out that this is outdoors so there is no ceiling. I've made it this way so that the field may work with as many types of Pokemon as possible, and challengers will be encouraged to use as much of the field as they can.
Outside the battlefield (the battlefield doesn't take up an entire city block) consists mainly of flat dry dirt terrain with various fighting rings and training obstacles for the gym Pokemon to use in their training. There will also be a 4-5 meter flat sandstone looking wall surrounding the perimeter of the gym, and red torii gate entrances on all four sides of the gym. Spectators of battles will stand behind the challenger on about a 2 meter high platform with a hand rail. A lobby area can also be found in a corner of the gym where Pokemon will rest, sleep, and be treated using natural herbs when injured.
Gym Pokemon Include: (These are Pokemon who live and train at the gym, and differ from those used in battle)
Mankey, Poliwag, Machop, Tyrogue, Shroomish, Meditite, Chimchar, Riolu, Croagunk, Scraggy, Mienfoo, Pancham. *Only basic forms are listed, but their evolved forms are included as well.
Gym Rules: Challengers must have at least 5 Pokemon to challenge this gym. To earn the Titan badge challengers must win two rounds of battles. The first is a 3v3 followed by a double battle. For the challenger to win the 3v3 they must defeat 2 of the leaders Pokemon whereas to lose, all of the challengers Pokemon must be defeated, and both sides are permitted to use substitutions. If the challenger wins the 3v3 they will then move on to the double battle where each side will use only two Pokemon (must be different from the ones used in the 3v3) and the first side to defeat both of their opponents Pokemon will be the victor. Upon the challenger winning the double battle match they will receive the Titan Badge.
Pokemon used against challengers with less than 3 badges:
Mankey -------------------- Cross Chop, Assurance, Thunderbolt, U-turn
Meditite -------------------- Force Palm, Confusion, Detect, Ice Punch
Mienfoo -------------------- Drain Punch, Payback, Poison Jab, Fake Out
Pancham -------------------- Circle Throw, Dark Pulse, Iron Head, Rock Tomb
Croagunk -------------------- Poison Jab, Revenge, Sucker Punch, X-Scissor
Pokemon used against challengers with 3-5 badges:
Machoke -------------------- Vital Throw, Knock Off, Dual Chop, Smack Down
Scrafty -------------------- High Jump Kick, Crunch, Poison Jab, Torment
Monferno -------------------- Mach Punch, Flame Wheel, Rock Slide, Shadow Claw
Hitmontop -------------------- Rolling Kick, Endeavor, Gyro Ball, Pursuit
Gallade --------------------Psycho Cut, Close Combat, Stone Edge, Leaf Blade
Pokemon used against challengers with more than 5 badges:
Blaziken -------------------- Sky Uppercut, Blaze Kick, Brave Bird, Stone Edge
Breloom -------------------- Dynamic Punch, Seed Bomb, Iron Tail, Stone Edge
Poliwrath --------------------Hydro Pump, Circle Throw, Hypnosis, Blizzard
Lucario -------------------- Aura Sphere, Flash Cannon, Bone Rush, Dragon Pulse
Mega Gallade --------------------Psycho Cut, Close Combat, Stone Edge, Leaf Blade
I decide in the moment which Pokemon to use during 3v3 or double battle matches, there is no order