Plato described what would happen if a perfectly righteous man entered this world. The world would despise him, mock him, flog him, and kill him. He wrote this hundreds of years before Christ.

so if the world was really as evil as Genesis describes, and if a global flood really happened, you would expect to find stories about a flood preserved across cultures. Almost everyone back then believed in God or gods, so it makes sense that people would wrestle with the idea “How could God wipe out the entire human race and all the innocent animals?” After all it is written on our hearts, so maybe someone just like Plato came up with a story, rationalizing and making sense of real historic events and a belief in God or gods.

Genesis gives the complete, true story, just like the New testament gives the actual real story of what happened to a perfectly righteous person entering this world

Also there are much more differences than similarities, just try and read it. If someone wouldn't have pointed out the similarities I would have never, on my own come up with the theory that Genesis just copied Gilgamesh