In terms of Gilgamesh/Enkidu - ah, I know there are several assyriologists in fandom who could probably correct me on this - but while as far as I know there's no erotic G/E (that survived from the ancient world), there is very little poetry surviving from that period at all. What we do know is that there are some older versions of some of the stories in the Epic which survived as fragments that don't reference Enkidu as a central figure, so that the whole Epic of Gilgamesh itself seems to be pairing-centered 'shipper fic that takes an originally not-major character and makes him the king's beloved. :D