Bhagavad Gita is a dictate, sermon given by Lord Krishna to Arjuna but intended for entire mankind. Bhagavad Gita deals with spiritual principles of life. One can follow Bhagavad Gita to reach state of enlightenment and finally moksha salvation. No other scripture is necessitated to become an enlightened one other than Bhagavad Gita.
Upanishads are independent spiritual treatises that evolved between Vedas (Hinduism revelations) and Bhagavad Gita. The prime reason for summation of Upanishads is the four Vedas were voluminous in nature, even difficult for scholars to decipher. To reduce this burden on scholars and masses different sages and Rishis compiled Upanishads.
All Upanishads deal with different spiritual disciplines but in depth. The messages contained in Upanishads were as vital as jnana wisdom contained in Bhagavad Gita. The 11th primary Upanishads form core of spirituality along with Bhagavad Gita. To explain individual precepts of spirituality Upanishads had a great role to play.
One indulging in Bhagavad Gita can also relate to principle Upanishads that had a different story to convey. Some of the narratives in Upanishads are in form of a story to make the essence clear.
To understand spirituality in depth Upanishads also form great reading. To expedite spiritual journey if one refers Upanishads along with Bhagavad Gita, it was an added advantage.
Everything from origin of cosmos when God Almighty, Brahman explodes self to dissolution of cosmos (Pralaya in Hinduism) is explicitly detailed in Upanishads.
For indulging in Bhagavad Gita and Upanishads, knowledge of Hindi and Sanskrit is definitely a great advantage. Authentic commentaries on Bhagavad Gita and principle Upanishads are available from Gita press, Gorakhpur, India still they are commentaries. They have limitations. Commentaries are a reflection of intelligence of the commentator, one who has yet to reach state of enlightenment.
All the commentaries on Bhagavad Gita and Upanishads must always be treated as guidebooks nothing more. The actual inner journey has to be undertaken by us indulging in contemplation (chintan) and in that none can help us.