Gilgamesh crosses the waters of death to ask one question: how did you obtain the eternal life you have. Utnapishtim's answer is the whole point of the episode. The gods held an assembly, made a single decision about one man, and will not be holding it again. Immortality was not achieved and cannot be repeated.
He then offers a test that is deliberately unfair. If Gilgamesh cannot conquer death, let him first conquer sleep, and stay awake for six days and seven nights. Gilgamesh sits down and is asleep almost at once. His wife bakes a loaf for each day and lines them up beside him, so that when he wakes and insists he only dozed, the row of loaves in every stage from fresh to mouldy is the evidence against him.
As a parting gift he is told of a thorny plant on the sea floor that restores an old man to youth. He ties stones to his feet, takes it, and carries it back to share with the elders of his city. Bathing at a pool on the way, he leaves it on the bank, and a snake takes it and sheds its skin as it goes. He sits down and weeps, and then goes home and shows the ferryman the walls of Uruk, which is where the epic began.
The eleventh tablet re-entered the modern world in 1872, when George Smith, working on fragments in the British Museum, read the flood account and announced it. The similarity to Genesis was immediately obvious to everyone, and the relationship between the accounts has been argued ever since, without anyone disputing that a relationship exists.