I read somewhere that Bible already contains almost similar teachings of Bhagavad Gita. Then what is so great about Gita?

It is quite fine for a Christian to eulogize Bible, do a deep study of it, absorb its spiritual guidelines and practice them in life. But to compare Bible with Bhagavad Gita, it requires quite a lot of spiritual background and intellectual capacity and a well balanced heart and head.

All religions have some common fundamental aspects which will cut across all religions and be applicable for the well being of the human race (like the importance of belief in God, a sense of surrender to the divine will, love, sharing, forgiving, sacrifice, need for humility and so on). You will definitely find similar ideas between Bible and Bhagavad Gita. But that does not mean the rest of Bible or rest of Gita can be discarded.

Bhagavad Gita covers so many facets of religion and spirituality — the concept of Atman, its deathlessness, karma and rebirth, need to follow one’s swadharma, the varnashrama differences, the concept of trigunas (Satwa, Rajas, Tamas), the qualities of people who attained steady divine wisdom, the nature of people of demonic tendencies, the practice of Karma yoga (doing one’s work without expecting fruits), the concept of Bhakti, the concept of Gnyana, the importance of total surrender (saranagathi), the all pervading nature of God, God being the indweller in all, the unity of Atman and Paramatman and so on.

I don’t think you can find equivalents in Bible to these concepts which are mostly specialties of Hinduism. In the spirit of Bhagavad Gita, every seeker can potentially become the Holy Trinity (The person – his spirit – God becoming one) whereas Bible will not accept such a thing. Only a messiah (Jesus) was fit enough for this.