Basically there was some major geological **** going down and yeah there were big floods, tons of people died, and an rich guy made a big boat and brought a bunch of animals. But the flood wasn't global. The animals on the ark weren't all animals from allover the earth, they were local, maybe within a thousand miles or so. It was a traumatic event, and that shows in the story as the writer refers to the whole world being covered in water. That's not literal, that's the writer's point of view, because the writer probably never knew of anything outside the Mesopotamian area. People usually didn't back then.
There's no indication that the Epic of Gilgamesh was intended to be a literal technical account of an event. Something big did happen, but it wasn't a literal global flood.