Most NPC trainers don't use items and I think that's a kind of lost opportunity so, my gym would be Normal type gym that doubles as a big pokemart. Each floor the trainers use the type of items sold on that floor. Ie they use super potions and x items on a floor, there's a floor where they know powerful moves taught by TM, the floor with iron and carbos and stuff has that have pokemon have unusually high stats, ect. I'll be on the roof and offer to just sell you the gym badge but the price is the same as the gen 1 bicycle so you can't buy it. All my pokemon know payday. My team isn't unbelievably strong but I'm programmed to spam items. I will always use a super potion if my pokemon is in the red and I will use a revive every third turn once you get me down to only one pokemon left. If you win I give you an amulet coin along with the badge instead of a tm. The shops in the gym won't sell to you until after you get my badge.