What is the main theme of The Epic of Gilgamesh, and what is the historical background of Gilgamesh? Is it a believable story?
The historical figure Gilgamesh, if he existed as many scholars believe, was a Sumerian king who is thought to have ruled over the city of Uruk circa 2800-2500 B.C. (in modern-day Iraq).
The Epic of Gilgamesh is incomplete. Fragments of tablets of the epic date from 2100 B.C., and what is widely read today is a nineteenth-century translation.
The epic contains many themes consistent with man's journey through life; it could be said that the earliest tablets are concerned with man's desire for power and how he faces limitations imposed by the gods. The relationship of Gilgamesh and Enkidu could be seen in psychological terms as the struggle between the id and the ego. Many of the epic's stories correlate with biblical stories (i.e. the deluge) and are also reminiscent of episodes from Homer's The Iliad and The Odyssey.
The overarching theme of the epic centers on man's rejection of his mortality and his consequent desire for immortality. Since life as man knows it is not eternal, if he wishes to be remembered, it must be for kindness and love, not the attainment of the power to oppress others.
To answer the last part of your question, if one reads the epic's episodes as metaphors, it is "believable." The acquisition of wisdom and acceptance of one's humanity are lessons that apply to us all, even if some of the trappings of the stories challenge our credulity.
The main theme in The Epic of Gilgamesh is that wisdom and kindness are superior attainments to immortality, and immortality may symbolize all self-centered attainments such as strength and power. The story of Gilgamesh is founded on the life of an historical ruler in a kingdom called Uruk, near Sumeria, in circa 1600 B.C. The tales began as oral tales passed down from one highly trained and skillful story teller to the next generation of equally trained and skilled story tellers, who were considered the keepers of a culture's greatest historical figures and achievements.
Historians don't doubt the authenticity of Gilgamesh as an historical ruler, and the tales were first written down by Sumerian scribes in 2100 B.C. Though the standard opinion is that mythological figures such as gods and other immortals are untrue legends, there are those who believe stories of mythological gods represented true individuals and events, as propounded by Robin Lane Fox of Oxford University in Pagans and Christians (1987).