The fifth gym in Platinum (fourth in Diamond and Pearl) hasn't been changed that much, but the Trainers have souped up their Pokemon's levels now and have changed their teams around a little bit. Crasher Wake is tougher than before and will be a tough fight for your fifth Badge. After beating him, though, you still need to chase after the Galactic goon, but the scene has gotten a bit different there, too. Oh, and you no longer get Defog inside of the Great Marsh, so there's no real reason to go in there now unless you want to catch some Pokemon (there are new Pokemon in there).