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Ghosts, I think they're neat, and cute.
My gimmick would be being an in general annoying prick with paralyze, hypnosis, etc, and just in general moves that are infuriating to deal with, such as Perish Song.
I'd instruct one or two trainers to try and have their last pokémon use Perish Song just to be a dick. No trainer will have less than three pokémon, and they'd all have at least one Super Potion.
Level tiers would be 30-40 for trainers, 40-50 for me. I'd have five pokémon just to draw things out.
I'd like the to make the gym itself a bit like the mansion in Resident Evil, or the castle in Resident Evil 4, though of course, it would be significantly smaller in size and scope.
Alternately, the gym could be a haunted ship in the harbor.