The gym leader chooses what platform they will battle on (Showdown or 3ds). The gym leader will have a mono-type team based off what gym they chose, excluding 2 pokemon they can choose to help with coverage and all that. The Challenger can bring whatever pokemon they want that is OU or below, excluding "special bans". The gym leader cannot change pokemon but they can change the movesets and items and even megas if they have more than 1 potential mega on their gym team. You can only challenge each gym once per day. If you do not beat all gyms within 2 weeks of beating the first one, you must battle all the gyms again.
Special Bans: A "special ban" is a rule that a gym leader can add that bans one specific pokemon OR ability that hurts their team really badly. An example of this is if the grass gym bans Talonflame, or if the dark gym bans Pixalate. There can only be 1 "special ban" in place per challenge.
Conclusion: Ultimately this is to have a good time around the chat and to bring a little competitive scene to the environment. Keep this in mind before challenging someone because I will not appreciate poor sportsmanship. The first person to beat all 8 gyms will win a prize of sorts, one I deem worthy of being the first to beat all 8 gyms. If no one beats all 8 gyms within the end of March, the gym leader with the most wins shall receive a prize of equal value. Gym leaders must report to me (blt) with their wins or losses and provide a replay (if on showdown) or the battle video codes using the Vs Recorder (if on 3ds).