In the dogfight between Gilgamesh and Berserker, Gilgamesh rides a golden fighter plane that is capable of hovering and flying at great speeds, supposedly by harnessing some "strange green power" (see the opening wings and glow when Gilgamesh prepares to dodge Berkserker's approach). The Gate of Babylon is supposed to contain Gilgamesh's treasures of his time, so how did such a futuristic contraption come into his possession?
This "airplane" is the Vimana. It is recorded in several epic Hindu texts, including the Ramayana and the Mahabharata. In fact, in the original Sanskrit works, there is more than one vimana, but in any case it is a flying contraption that gods would ride. There is no record of them in Babylonian literature as far as I know, but of course Gilgamesh owns the prototypes for all of the technology that appeared in later cultures, which presumably includes this.
There are several examples of what would seem like futuristic technology in ancient cultures like the Egyptians and Babylonians. In temple of Seti I there are hyroglyphics that depict blimps and helicopters.
The Vimana is another example of this. Also Ea is a construct "not of this world" and is an Anti - World Noble Phantasm that defies the analysis of Unlimited Bladeworks despite not being a Divine Holy Relic like Excalibur. This suggest that Vimana and Ea are of Alien Origin.
This may come into play later as Fate is part of the Nasu-verse and we know they use the many worlds interpretation of time and space, adding or hinting at alien worlds to this is not a stretch for them.
Basically, because Gilgamesh was the very first king, every treasure was claimed by him, including everything made by Man. Even after he died, things than Mankind dreamed up afterwards were claimed by him. That or the power of magic back then meant that his kingdom was able to make magical things equal or greater to modern inventions. It is also is that as the very first Hero, he must have the prototype to every weapon or item used by later heroes. The only things Gate of Babylon doesn't contain is anything made with new concepts by a new human race, or items made by extraterrestials or with extraterrestial knowledge. In Fate/Extra, he even has a spaceship able to move at the speed of light.
The article I got this all from is worded a little confusingly, but I think it is that if Mankind has or will make something, then as the first King and Hero, he already has it in his vault. So either the Vimana is a magical airship, or it's based on a ship Mankind will make.