Gilgamesh turned and glared at Mia as he spoke. "...who do you think you are speaking to, Mongrel?" he sneered. "It does not matter what sort of petty tricks you attempt to use against me - to think that you attempted to undermine the King with mere trickery - know your place." 

Unfortunately for Mia, there was a severe difference in most abilities and the Gate of Babylon. While most servants would have to extend their power to summon their noble phantasm; the weapons of their legend, Gate of Babylon worked differently. Gilgamesh himself did not create Gate of Babylon, no, he did not have the ability nor capacity to create it. To attempt to "recreate" the Gate of Babylon would instantly drain even the most prana efficient caster servant. Instead, his Gate of Babylon was something that simply "existed". It was the King's treasury, something from which every single legendary weapon was passed down from. It was something that Gilgamesh did not create; rather his very existence was defined by his ability to access it.

While it did cost him much more exertion than he was used to - something he would have the mongrel pay dearly for - he opened the gate of Babylon. Even with his ability drained to that of a normal person, there was still some thing that ensured he still had enough mana reserves to summon the key. 

His Magic Resistance. Even though it was quite low, it was till enough for him to prevent pandora's box from draining that miniscule amont of prana required. Once it was opened, it was a floodgate of power available to Gilgamesh. Behind him, weapons of mighty powers, heroic and legendary weapons from the entire human history appeared behind him. To his left, the Jingu Bang, the great staff of the monkey king sun wukong appeared. Beside it, Caliburn, the sword which King Arthur drew from the stone to become king began to appear from the gate. 

Gungnir, the spear of Odin which was said to strike with the power of lightning itself.

Dijun's fury, the nine arrows which the legendary archer Houyi used to shoot down nine suns. 

Gae Buidhe, the yellow rose of mortality; the spear wielded by the Irish warrior Diarmuid Ua Duibhne, a weapon which inflicts a never-healing wound upon it's victim. 

They were but a few of the many Gilgamesh pulled out. Each weapon would be enough individually for a legend. Together, all in one treasury, they were fit for the King of Heroes. Already, with the power of so many legends, with countless weapons capable of shattering mountains, of rending reality, of reversing the very flow of time being pulled out, Pandora's box began trembling; lightning forming across it's sky as it began to be overwhelmed. 

And Gilgamesh had yet to even utilize Ea. 

"Lament your insolence and foolishness in attempting to entrap the King and die, mongrel." Gilgamesh proclaimed dramatically as he fired the Jingu Bang and Gae Buidhe at Mia.