The Upanishads or the Vedanta expounded the science of human possibilities a thousand years earlier (than the Gita), and the Gita expounds the practical application of that science. Hence, Swami Vivekananda considered the Gita as "The best book of practical Vedanta".

Reference: Universal Message of the Bhagavad Gita Vol. 1 by Swami Ranganathananda (p.13)