Not only Bhagavad Gita has all the answers necessitated to reach god but the deadly combination of teachings of Bhagavad Gita and 11 principal Upanishads (independent a spiritual treatises) form core of entire spirituality contained in Hinduism.
Knower of Bhagavad Gita and Upanishads gets to know everything relating to God right from stage of big bang to dissolution of entire cosmos (Pralaya in Hinduism).
Beyond these 12 scriptures there is nothing worth knowable in entire cosmos. Indulging in these scriptures human beings finally gained emancipation from cycle of birth and death becoming a pure soul atman at the end of cosmic journey.
The liberated soul atman can finally venture to any part of cosmos at will. Before human beings reached state of enlightenment followed by moksha salvation the soul atman was bonded to solar system. It becomes free from earthly bondages only when one gains Mukti, liberation.
The 700 shlokas verses contained in Bhagavad Gita detailed every single aspect of spirituality necessitated to reach state of enlightenment, self realization, god realization.
The journey of spirituality truly makes one become a pure soul atman, wow!
How we can demolish our ego, how to establish control over five senses and mind, how to practice absolute celibacy for a minimum period of 12 years in continuation, full awakening of kundalini shakti… all is explicitly detailed in Bhagavad Gita.