Gym itself was an old run down cram school in the town being squatted in by a group of "reformed" Grunts, Punks, and general ne'er-do-wells. The merry band of rogues is lead by the Gym leader an over the top costumed vigilante that's trying to give former Thugs and other seedy elements a second chance to do good.
The group volunteers in the town, donates, rescues Pokemon, and keeps the area safe from actual seedy elements with their brand of Diet Seedy elements, a bit of antics, and wacky shenanigans. They genuinely do want to do good in the world, but old habits die hard as it were so sometimes they need reigned in. A lot of the trainers in the Gym the main character must battle to get the leader also use Dark-type Pokemon. Gastly's line is also used.
Gym's gimmick is that any and all hazards set in one battle persist to the rest unless the main character clears them with either a Rapid Spin or several devices scattered in the Gym to clear them. Lots of status conditions used by the trainers and leader. An element of the gym is a simple maze to get to the leader, but often the trainers in the gym, upon engaging in battle with the main character and walking to them to battle, block the most obvious path and making the player backtrack and take a different route. This symbolizes the trainers in wanting to do good things but ending up an annoyance to the player.
Upon victory the player is awarded TM06, Toxic, and the Justice Badge. It is a series of 3 purple circles side by side with smaller black circles in them, with small white rings in the black circle.
A post-game side quest involves going through a small building full of enemies with a named NPC antagonist as your battle partner as they have been double crossed. Throughout the encounters in the building, the NPC talks at the player and works through their issues that drove them to be a grunt. They join the gym and award the player a Toxic Plate.
https://www.youtube.com/watch?v=GSHjicBRDOMr8/h8
