Bhagavad Gita should be make national book because it teaches us the principles to live a healthy life. Moreover Bhagavad Gita as a standalone book doesn\'t represent any religion. It is universal, independent of any religion and anybody can adopt its principles.
On the other hand hindus had accepted the principles of this book ,so that doesn\'t make any sense that it is a hindu based religious book. If other religious wants they can also accept it.