Hi Vishal,
Booking is mandatory for entry to the trekking trail. The Karnataka Forest Department has implemented this system to reduce crowding and protect the environment. If you try to enter Skandagiri without booking, you will not be allowed to trek. There is no guarantee that entry will be allowed at any time, even if you are willing to pay the entry fee. I recommend that you book your tickets in advance, especially if you are planning to visit on a weekend or during peak season. You can book tickets online through the Karnataka Eco Tourism website.