I'm going for a mized-type Gym, as I don't like the vulnerability of training one type.
My gym would be elegant in design, with columns, granite/marble floors, lots of dark wood furniture, and the like. Jazzy piano music would be playing in the background and I'd have a statue of the legendary trainer Leaf gracing the entrance. I'd pose some sort of intellectual challenge, like a quiz. Subject of the quiz to be determined, but probably some of my own oddball interests like Quality Inns or large American luxury cars of the 1970's and 1980's... And you must face trainers if you get the quiz wrong, similar to Blaine's Gym.
Persian, Jolteon, Metagross, Walrein, Weezing, Flygon.
I'd probably have all girl trainers in the gym under me Mostly Ladies, Parasol Ladies, School Kids, and Picnickers.