I already beat the game and pretty much visit every place to visit besides Routes 229 & 228 and Spring pass with Turnback cave. I was wondering if I can battle all the Sinnoh Gym Leaders again since I saw on this website the Gym Leaders themselves using higher leveled Pokemon with better movesets. I am sorry for being wrong if you can't battle them again. Please help me! Thank you so much! :)
You can only rebattle them in Pokemon Platinum.