Did I post before? If not, then I'll post now. If I was a gym leader it would probably be Dark type or Dragon type.
1) If it was Dark type, my theme would be a tall fortress surrounded by skulls, pit-holes, thrones etc resembling a necromancer's lair. The puzzle to it would be to collect all the essentials required to summon all sorts of undead like zombies, skeletons, wraith etc and fighting them in a pokemon battle. Then a throne room would unlock in which I would be sitting on the throne and challenge you. My pokemon team would be a Justified Absol w/ Choice Scarf, Hydreigon with Roseli Berry, Protean Greninja @expert belt, Spiritomb @leftovers, Cacturne w/Focus Sash and my ace Yveltal @Life orb. I know legendaries are cheap but Yveltal is related to death and destruction, which is the theme of this gym.
2) If it was Dragon type, the background of the theme would be a dragon's base deep inside a fiery volcano and you have to walk on bridges and rocks, figuring out ways to cross the burning magma. Also, if you are not careful, your pokemon can get hit by small burst of ash and lava. I got inspired by Serra's gym in which if you break the wrong mirror, your pokemon get hurt. My team would be comprising of Moxie Salamence w/Life Orb, Multi-Scale Dragonite with leftovers, a Gooey Goodra with leftovers, Flygon with Lum Berry, Hydreigon w/Choice Scarf and ace Rough-Skin Garchomp with Rocky Helmet.