The Epic of Gilgamesh is one of the first of a variety of things. It is the oldest written story ever told. As such, Gilgamesh is the very first literary hero. To clarify, the Epic is the oldest story that we know of. It is extremely possible that there was a variety of legends and stories before Gilgamesh, we simply haven’t come across it, or are able to read it. If you have the time, you can read a longer summary of the Epic here.
To begin with, the story takes place in Uruk, which is also considered to be the first true city in the world, believed to have been founded in 4500 BCE, and was once the most important city in ancient Mesopotamia. In fact, there are so many firsts that took place here that you should totally check out an awesome page on Uruk here.
The story follows a god-king named Gilgamesh, who was a tyrannical king to his subjects, committing murder and rape whenever he felt like it. The gods got mad at this and sent an wild man named Enkidu to kill Gilgamesh. They met, and fought, and like many bar brawl stories that we have heard, they became best bros. Again, the gods got mad and killed Enkidu. Now grieving for his friend, and facing his own mortality for the first time, Gilgamesh went in search of immortality, and found it. This process takes a couple years, but it ends up making him a much better person.
At the end, one of the main themes of the story is a form of self redemption, as we see a god-king go from being a rapist murderer to maybe being a much better person. A much larger thought behind it is that Gilgamesh is a template for heroes throughout fictional history. Writers follow this template for millenia, from movies to books to TV shows. As such, it might be useful to look into the very first hero when you write a hero.