The Gita is like a summary of the various philosophies existing at the time in ancient India 5000-6000 years ago at the end of the Dwapara Yuga.
Krishna is revered as the Jagadguru or teacher of the world, because he summarised all the different philosophies in conversation with Arjuna.
Interestingly the author of the Mahabharatha in which the Bhagavad Gita is found.. is Veda Vyasa or the one who divided (ordered, collated) all the Vedas. His real name was Krishna Dwaipayana. The other name for Draupadi, the heroine of the Mahabharatha is also Krishnaa.
Mahabharatha is an Itihasa or historical epic. However in writing this, Veda Vyasa wanted to explain all the Vedic and Upanishadic truths to people in a story form because ordinary people can understand stories better than philosophic jargon. It is said that everything that is there in the world is in the Mahabharata.
The best English translation of the Mahabharata is by Kisari Mohan Ganguli. A free online version is available at Sacred-Texts.com