With a backcountry permit for Sleeping Giant Provincial Park, you can camp anywhere in the backcountry. There are about 40 backcountry sites in the park. Unfortunately, you can’t reserve a permit ahead of time. Backcountry permits are only available on first come, first serve basis. If you worried about the availability of the permits, you can spend the first night at the organized campground. That way you can be in the park office first thing in the morning. Frontcountry campsites can booked online: https://reservations.ontarioparks.com/SleepingGiant
Hope it helps. Let me know if you have any further questions.