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.