Where is the Strinton gym?

There no place in Pokemon Platinum called Strinton gym. There is however a gym in Pokemon Black and White called Striaton Gym though. Its the first gym you get to in Pokemon Black and White. I'm guessing you got the two games mixed up.

In no point of Pok�mon Platinum is there a gym called Strinton gym. However, in Pok�mon Black and White, there is a city called Striaton City, and since the city has a gym, you may have gotten Pok�mon Platinum mixed up with the two games. The three gym leaders of Striaton City are Chili, Cress, and Cilantro, who give you the Trio Badge after defeated. Which one you battle is depended on which was your first Pok�mon. If you pick Oshawott, you have to battle Cilan, if you pick Snivy, you have to battle Chili, and if you pick Tepig, you have to battle Cress. As you can see your first Pok�mon will not be effective to the gym, so in the Dreamyard, a girl gives you either Pansage if you picked Tepig, Panpour if you picked Snivy, and Pansear if you picked Oshawott so you may have a chance with those Pok�mon. Cress uses Panpour and Lillipup. Chili uses Lillipup and Pansear, and Cilan uses Lillipup and Pansage. You get the Pok�mon from the Dreamyard at lv. 10. Raise it to lv. 13 or higher.