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Poison type gym, mid game.
The gym itself would be filled with haz-mat workers that use mostly poison types and pokemon fitting the haz-mat theme. The gym would be a maze and every step you take does poison damage to your whole party, regardless of their status or type.
When you reach the gym leader, your whole party is automatically healed. The leader is wearing a gas mask and you can't tell what they look like. They talk in muffles the player cannot make out.
The team is various poison types with an emphasis on toxic and venoshock.
Once defeated, the gym leader takes off the mask, revealing a pretty normal looking girl. She tells the player it's important to love all pokemon, even the ones made of trash and poison sludge.
The player receives the TMs toxic and return.