Well, I would have a fire gym, in a volcano shape. All the battles would be double, so it wasn´t going to be easy. There would be 6 trainers, not only using Fire Pokemon, but Pokemon that could learn Fire moves. Also my gym would be a resistance chamber, where your Pokemon can improve their focus by meditating on the geysers
Cooltrainers Marcos & Rocio
All Pokemon LV30
Combusken
Magcargo
Granbull (can learn Fire Punch, Sunny Day, Flamethrower, Fire Blast and Overheat)
Clefairy (can learn the same attacks, except Overheat)
Jigglypuff (can learn the same attacks)
Nidorina (can learn Sunny Day)
Fighters Gonzalo & Julian
All Pokemon LV35
Steelix (can learn Sunny Day and Flamethrower)
Charmeleon
Rhydon (can learn Fire Punch, Sunny Day, Flamethrower and Fire Blast)
Shiftry (can learn Sunny Day and Nature Power transforms into Overheat in the Gym)
Snorlax (can learn Fire Punch, Sunny Day, Flamethrower and Fire Blast)
Solrock (can learn Fire Spin, Fire Punch, Sunny Day, Flamethrower, Fire Blast and Overheat)
Rich Boy Lucas and Clown Maxi
All Pokemon LV40
Flareon
Charizard
Typlosion
Spinda
Ursaring
Aipom
Then, the last battle of the gym:
Blazing Leader Matias
All Pokemon LV50
Blaziken
Flareon
Snorlax (can learn Fire Punch, Sunny Day, Flamethrower and Fire Blast)
Charizard
Houndoom
Ludicolo (Nature Power=Overheat)
As the battle is on a Lava River (The Pokemon are on rocks) using physical attacks, except if you are Fire or Flying, does damage to your Pokemon. Also, Fire attacks increase its power by 10%. If you win, I would give you a TM with my favourite move, Flamethrower, and 5000 money