Is the Bhagavad-gita historical or allegorical?
An allegory is a story that can be interpreted to reveal a hidden meaning, usually moral or political. Allegories have their value, but we recognize the Bhagavad-gita as a work of nonfiction, intended for a specific purpose, and not open to interpretation by just anyone.
Our sources say Krishna factually spoke to Arjuna on the Battlefield of Kurukshetra, to enlighten him about spiritual reality; that the self is different from the body, that each of us is a unique, eternal, spiritual entity, endowed by our Creator with specific duties to perform in service to Him, both in this brief lifetime and throughout eternity.
Accepted as it is, the Gita is easy for anyone to understand and put to use. It has historical value, shedding light on the advanced spiritual culture of a bygone era. It contains practical advice for becoming free from the miseries of life and experiencing the natural happiness of the self. It has spiritual value as an introduction to the teachings of the Supreme Person, Krishna, on the nature of our eternal relationship with Him in this life and the next.
If accepted as an allegory, it's unclear whose allegory it might be and what their intention in writing it may have been. Some may like to speculate about this, but we'd consider such questioning useless, since the Bhagavad-gita has such immense value when accepted as it is. If it's accepted as simply a "story," then what is its value, except to whomever whimsically chooses to "believe" it?
We accept the Bhagavad-gita not as story, or even as a theoretical philosophical work, but as the blueprint of a spiritual culture, a culture with extremely high moral and spiritual standards, into which the Bhagavad-gita was spoken, and which perpetually exists wherever and whenever its teachings are followed and practically applied.
In the Gita itself, Krishna recommends one learn His teachings from a living representative of the tradition, to get the greatest possible benefit from the literature. As Srila Prabhupada mentions in his own commentary on the Bhagavad-gita, Bhagavad-gita As It Is , it is meant for the formation of character, not as an abstract philosophical treatise meant for armchair speculators. If accepted as it is, the Gita can help one have a profound spiritual awakening—Krishna says that by careful study, we can know both ourselves and God factually. It is doubtful whether the study of any fiction—no matter how imaginative and seemingly full of meaning—could promise and deliver tangible results of such magnitude.