He transmigrates as an unborn Gilgamesh.
As an adult, he defies the gods of the Age of Gods.
But after finding the secret to immortality, he discovers that the gods who drove the immortal away had tampered with it. His body can only last five years. Then, his innate Noble Phantasm transmigrates him to a later era, to the moment Artoria of Britain has just pulled the sword from the stone. Merlin brings him to Artoria.
Artoria keeps him hidden. He guides her toward the path of kingship and becomes her friend, learning everything about her, but Gilgamesh never tells her who he really is.
His body, however, is failing. As he nears death, he tells her the truth, and spends the rest of his days by her side.
Then they meet again during the Fourth Holy Grail War.
No System. Far stronger than in the original. A crossover.
This novel and its characters are entirely fictional. Any resemblance to reality is purely coincidental. Do not imitate.
What a pity, what a good subject matter it is. . .