at the end of the Magician, in the preview for the Sorceress, it states "Gilgamesh also knows all the elemental magics, thought strangely, he never possessed the power to use them. The King has no aura."
I also think there is something different about how Gilgamesh is immortal. Since the other immortal humans are immortal - but not indestructible ("“You’re immortal, William, not invulnerable.” Machiavelli smiled." pg. 463 of the Sorceress; "Nothing—Elder, Next Generation,
immortal or human—was completely indestructible." pg. 76 of the Sorceress) - how is it that Gilgamesh is "impossible to kill" (Pally says that on pg. 459 of the Sorceress). We are told that the reason Gilgamesh was locked up in the 40s was because he wanted to be underneath the atomic bomb when it was set off because he figured it was the only way he could die.
So, got to figure out what the difference is....