In earlier era in the erstwhile Bharatvarsha (now India) highly evolved Sages and Rishis indulged in Vedas to reach cosmic end of life, the state of self realization, god realization. Until that time the human brain had not yet fully evolved. The jnana wisdom contained in Vedas was limited to sages and Rishis, even scholars finding it difficult to decipher.
With precipitation of jnana wisdom contained in Bhagavad Gita indulgence in Vedas (Hinduism revelations, the first words of God to mankind) is not necessary. Everything needed to reach stage of self realization is contained in teachings of Lord Krishna in Bhagavad Gita. The 700 shlokas verses of Bhagavad Gita were pure wisdom having no parallel in mankind.
In present times only one scripture the sacred text of Bhagavad Gita suffices for mankind to reach state of enlightenment, self realization, god realization.
Indulgence in Upanishads (independent spiritual treatises) is an added advantage and expedites the spiritual journey but is not still necessitated. One can do without jnana wisdom contained in Upanishads and Vedas.
Ever since sermon of Bhagavad Gita came into scene, all seekers who succeeded in reaching state of enlightenment indulged in teachings of Bhagavad Gita alone. Adi Shankaracharya travelled this path and finally wrote a commentary on Bhagavad Gita in Sanskrit. Even Sri Ramakrishna Paramhansa and Maharshi Ramana followed dictates of spirituality contained in Bhagavad Gita. Next to follow was Swami Vivekananda.
Without an iota of doubt, seekers wanting to reach God in their lifetime must indulge in jnana wisdom contained in Bhagavad Gita to reach stage of self realization, god realization.
But we must remember one thing. Everything in Bhagavad Gita can be understood only by indulgence in contemplation (chintan), never literally.