I truly hate that this sort of thing is so prevalent on webnovel. As a writer myself, it really pisses me off to know that not only are there people out there who are willing to steal the content you put hours into making, but also attempt to monetise it? Every chapter is finished with a patreon link, which is completely within your rights to do as an author, but this isn't the real author is it? I think there are about 10 people subscribed to this guy's patreon waiting for chapters that are literally already posted on fanfiction. World just sucks man. Hopefully this gets taken down ASAP

They are Sumerian Myths, some of the earliest in recorded history. In a world like DxD, where the abrahamic factions basically hold all control over the supernatural, such things have faded into obscurity. Keep in mind, you only probably know Gilgamesh and Enkidu from their fate interpretation, not from the original tablet held in the British Museum, so all these fallen angels not knowing who she is isn't something I feel unlikely.

This is in the start of DxD canon, and your second question will hopefully be answered in the next chapter