The purpose of any prologue is to offer readers insight into the setting and deatils important to the backstory (also presented in the prologue). In some circumstances, the prologue offers readers an allusion (a casual reference to mythology, religious stories, legend, or historical event which alows the reader to connect prior knowledge to the text).
In regards to The Epic of Gilamesh, the prologue offers information about the character of Gilamesh. He is a demi-god (part god, part mortal) and defined as a superior being. The prologue also defines the story which follows as the story of Gilamesh (being written by his own hand).
Therefore, the main purpose of the prologue to The Epic of Gilamesh is to introduce the reader to the background of Gilamesh prior to reading his story. Readers must know what and who he is.