Overall, I did not have *too* many problems with the gym leaders.
On my Japanese Platinum, I had a bit of trouble against Fantina and Maylene. My only decent Pokemon against Fantina was Monferno, and he struggled against Mismagius. By the time I got to Maylene, I had traded over a Pokemon team from Pearl, but they were all unevolved and were at fairly low levels so they would listen to me. It took me a few tries in this gym, mostly because Lucario kept KOing all my Pokemon.
My English Platinum was easier, partly because I traded in my team at the earliest point in the game. (And this time, some of my Pokemon were fully evolved, such as Togekiss and Kingdra, even though the latter didn't obey) This team was also more balanced than my Japanese one, so the gyms were fairly average in difficulty.
What I found strange was that Byron was the easiest BY FAR. On both my games, I only had to use one Pokemon against him, whose was around the same level as his Pokemon. What's even more pathetic is that both times, my Pokemon did not lose a single HP.