The Potion of Youth (若返りの霊薬, Wakagaeri no Reiyaku?) is a Noble Phantasm that returns the user to their youth, stored in the Gate of Babylon. It is kept in a small, translucent bottle with a lustrous, beautifully-ornamented design made of an unknown material resembling china or crystal. It made from the herb of immortality that Gilgamesh had sought after the death of Enkidu. Seeking immortality to avoid his own inevitable death, he sought out the sage, UtnapishtimWP, in Kigal, but learned that the sage's immortality was obtained from becoming a god. Utnapishtim was changed into a half-plant in this process. Rejecting it because it would remove his desires as a human, the sage instead told him of the root of an herb that grew in the deep of Kigal that was the true secret behind immortality without seeking the mercy of the gods.
Gilgamesh found the herb and, for the first time in his life, felt joy and a sense of accomplishment. When he was taking a bath to return to Uruk in his majesty, however, this herb was eaten by a hungry snake. Having recognized the herb as lost, Gilgamesh eventually reached an epiphany that no longer required for him to seek immortality since he realized that the essence of humanity lied within the discovery of joy and the subsequent loss of that joy. Since the potion worked to reverse the drinker to its youth instead of as an elixir of immortality, the snake and its descendants shed their skin in order to bring back their youth. As it was still a rare treasure that he had planned to obtain even before his desperate quest, he returned to the deep to keep it as an exotic wonder to decorate his vault.
Drinking it returns him to the form of a child, drastically altering both his appearance and personality. It is used in cases where he would need it to tolerate and blend into the society around him, such as spending ten years in the modern world after the Fourth Holy Grail War. He can also use it in a case where he views something as a childish game like the False Holy Grail War, claiming that he would spend his time in leisure until an enemy worthy of his full strength appears. He keeps all of his memories, but he is disconnected from his child self due to the vast differences between their personalities. Though his child self knows of his later actions, he cannot comprehend his adult self's actions, and his adult self has little knowledge of his past. He has the ability to forcibly reverse the effects when facing an enemy he feels is worth fighting.
 Fate/side material - Encyclopedia: Gilgamesh [Servant], p.059 [T]
The Archer of the previous Grail War. Mankind's oldest king of heroes, who defeated Saber. For more details, refer to his status screen in the game.
As no proper Heroic Spirit can match this man, he could be called the "Servant Killer".
Without a doubt, he is the strongest existence amongst the Servants.
Though he lost due to obsession against Saber…
Though he lost due to pride against Shirou…
Though he lost due to carelessness against XXXXX…
…the fact of the matter is that if he actually gets serious, he is a Servant that cannot be rivaled.
Perhaps because he has been living in human society for the past ten years, he seems to be hip on the latest fashions. He owns various casual clothes, but his favorite is the biker outfit he wore in the Rin route.
Well, he liked it enough to overlook Shirou's group and leave instead of letting it get dirty, at any rate. I can't believe the protagonist's life was worth less to Gil than his coat…!
It might seem strange how Gil could live in human society for ten years with a personality like his and not cause a fuss, but Gil must have realized it would have been a problem, too, because apparently he drank a certain Noble Phantasm to change his appearance and personality.
Also, during the design stage, I selfishly insisted that "Gil absolutely has to wear full golden plate armor".
Why, you ask? Well, it might be because I was repeatedly challenging a certain 60 story tower at the time…
- Fate/EXTRA CCC - Gilgamesh Secret Garden Conversation