The thing about the Epic of Gilgamesh is that it's arguably the oldest extant example of a tale that's very loosely based on a true story; Gilgamesh is generally agreed to have been a historical king of the Sumerian city-state of Uruk... but anything beyond that basic fact is unknown.

While little is really known of the historical Gilgamesh, surviving Mesopotamian law codes such as the Code of Ur-Nammu, Laws of Eshnunna, and Code of Hammurabi don't support the idea of any actual kings having the legal right to rape their female subjects. While rape of women in these law codes was often treated as a property crime against a woman's father or husband (or owner if she was a slave), it was nonetheless a crime.

The text about the pharaoh Unas is, however, another ball of wax. Its context is a pyramid text honoring Unas as a living incarnation of the crocodile-headed fertility god, Sobek. So, we probably should be hesitant to take that passage literally. Unlike with the Mesopotamians, however, we can't look at any actual law codes, because no ancient Egyptian law code has survived.