First, there is strong evidence that the Book of Genesis was written by eye witnesses, including the sons of Noah. It clearly pre-dates the Epic of Gilgamesh...and is much more credible, given that the "Epic" is mythology, and Genesis 6-8 appear to be eye-witness accounts.
I am willing to provide evidence later to my assertion above.
The book of Genesis was not written by eyewitnesses ... Moses wrote Genesis ... and he was not there ...
now ... perhaphs Moses had eyewitness SOURCES ...
THAT is a different issue ...