I'd like to have a mixed type gym, but that would be horribly unbalanced so I think I might go with Ice to add some variety.
Gym layout- Ice plains, very little in the way of scenery, Maybe pillars made out of white marble (ice doesn't seem like a good idea for this), Once you make it through the first section you would enter into a second, slightly smaller room, this would be were I would be waiting for somebody to make it through to me (there would be ice stalagmite/stalagtites coming from the floor forcing people to fight gym trainers before they reach me). My chamber itself would be simple, with snow (artificial I guess) swirling around, an Ice sculpture of me with an epic cape in one side of the room and on the other side there would be a pool of water.
I would use Dewgong, Glaceon, Articuno (come on, legendarys have been appearing in the animie in the hands of young trainers, surely a gym leader could get one if they can) , Beartic, Walrein and Mamoswine.
Otherwise, I'd have an electric gym-
It would be set out like pewter gym was, except there would be steel pillars and a metal floor, and would have more trainers. Instead of boulders scattered around there would be power coils running along both sides of the room with two generators running at the back. Also there would be electric cables running along the ceiling.
I would use: Raichu, Magneton, Zebstrika, Zapdos (see Articuno note) , Galvantula and Luxray