Bhagavad Gita and Upanishads (independent treatises) of Hinduism are integrally connected. Practically all Upanishads deal with mystical… philosophical concepts detailed in Bhagavad Gita. After advent of Bhagavad Gita by Lord Krishna… different sages and saints of different era contemplated on precepts of spirituality detailed in Bhagavad Gita resulting in compilation of Upanishads! Every Upanishad rotated around a particular spiritual topic detailing everything relating to that subject.
Whatever is contained in Upanishads is also contained in Bhagavad Gita of Hinduism. But whatever is contained in Bhagavad Gita does not exist in single Upanishad or Veda. Upanishads truly are great philosophical treatises that independently deal with different aspects of spirituality contained in Bhagavad Gita!
For gaining enlightenment jnana (wisdom) contained in Bhagavad Gita of Hinduism sufficed. However, contemplation on jnana (wisdom) contained in various Upanishads was of great help. Hinduism consisted of many Upanishads… totaling almost above 180. But primary nine Upanishads were the most referred to. Some Upanishads contained examples that facilitated understanding of deep philosophical underlying meaning.
The primary nine Upanishads consisted of Katha Upanishad, Kena Upanishad, Isa Upanishad, Prashna Upanishad, Mundaka Upanishad, Mandukya Upanishad, Aitreya Upanishad, Taittiriya Upanishad and Shwetashwatara Upanishad. Apart from above nine Upanishads, Chandogya Upanishad and Brihadarankya Upanishad were also very important.
Every single Upanishad emanated after advent of Bhagavad Gita and were composed and written by an enlightened soul. Unless one gained enlightenment… discussing precepts contained in Bhagavad Gita was not possible. One who had absolute knowledge was competent to indulge in sacred texts resulting in compilation of various Upanishads (independent treatises).
Every single Upanishad is supportive of precepts of spirituality contained in Bhagavad Gita. Reading Upanishads along with Bhagavad Gita definitely helped spiritual travellers cut short the journey. Whatever was contained in Upanishads was as deep as jnana wisdom contained in Bhagavad Gita. Like Bhagavad Gita… the underlying meaning of precepts contained in Upanishads could never be understood literally. One had to understand the underlying meaning of precepts contained in Upanishads by learning to read in between the lines.
Lucky are those who in the early stages of life realized the importance of jnana wisdom contained in Upanishads.