Korean national park mountains except Bukhansan will be closed starting from 15-Nov to 15-Dec, and from 16-Feb to 15-May every year. During these days it is not possible to go up to the peak but only few area like temples. The reason for closing is to protect from the fire caused by dry weather.
Please make sure those days, if you plan to visit below mountains during those days.
5, Odaesan(including Gyebangsan)
Hope this Helps you.