Gate of Babylon

Gate of Babylon, the Treasure of the King, one of the major Noble Phantasms of Gilgamesh. A key-shaped sword that connects to the golden capital.

By connecting the space to the vault, he can pick the items in it as he pleases. Needless to say, the Noble Phantasm becomes more powerful with the wealth of the owner.

Similarly, there probably is a Road of Babylon.

Source: Fate/Side Material encyclopedia entry (p.57)

Gilgamesh mainly uses the Gate of Babylon to shoot the countless weapons in it like a rain of projectiles, but there also are various items in it, such as a mirror; Fate/Zero also shows us a flying machine. Gilgamesh can also picks the items one by one for specific use, of course.

The weapons are the prototypes of the more recent Noble Phantasms. While Cú Chulainn has the Gáe Bolg that became famous, Gilgamesh possesses a weapon that later became the Gáe Bolg, but wasn't famous at the time Gilgamesh owned it.

The weapons that are seen in the Gate of Babylon are the prototypes of Caladbolg, Dainslef, Durandal, Gáe Bolg, Gram, Gungnir, Harpe, Houtengeki, Vajra.