>>35502414A normal type gym but every trainer uses a different kind of weather with their team, with the gym leader making use of all 4 weathers as well as coverage moves in order to show off the effects of normal type moves like weatherball, natural gift and nature power etc.
Would be based inside a TV studio/weather report centre with the TM being for weatherball. Gym leader uses castform to further show off this effect.
This basically acts to try and teach the player about the effects of weather besides stand alone npcs and the importance and movepools of pokemon, ass opposed to just relying on STAB to charge through the game.