Youth Potion

"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."

Fate/side material - Encyclopedia: Gilgamesh [Servant], p.059

"Indeed, the herb of immortality was of little importance when compared to the truth that he had arrived at then.

“―――Well, after I brought Uruk to completion,
I stole back to the deep and retrieved it.
It was a rare treasure in of itself.”

Retrieved it?!

You’re not free of cravings at all, King of Heroes.....!

“Mm. Don’t say that.
Amassing treasures is like an instinct to me;
just wordlessly brush it aside.

‘Better safe than sorry,’ as it is said.
You never know, there may one day be a situation that I can only tolerate as a child.”"

Fate/Extra CCC