Platinum's gyms feature several changes from those in Diamond/Pearl. Gyms 3-5 switch their orders and most change their Pokemon to take advantage of the updated local dex. Now the gym leaders only use Pokemon of their specialist type - for example in D/P, Volkner used a Water type and a Normal type in addition to his two Electric Pokemon.
After beating the Elite Four and completing the Stark Mountain prologue, you can rematch gym leaders and the five trainers you partnered with during the game, at the Trainer Café in the Survival Area.