According to Guns, Germs, and Steel, what are the proximate factors responsible for the rise of the kingdoms of Gilgamesh and Cyrus?
Diamond's book does not explicitly address either of these kingdoms. Therefore, we have to look at more general ideas that are contained in the book to answer this.
According to Diamond, the proximate cause of the rise of any kingdom is agriculture. Without agriculture, kingdoms are not possible. Diamond argues in Chapter 14 that kingdoms come about when there are high population densities in a given area. When this happens, a more centralized state like a kingdom is necessary in order to maintain peace among the people. High population densities only come about, Diamond says, in places where there is enough agriculture to produce the surpluses needed to feed all those people.
Therefore, the proximate cause of the rise of any kingdom is agriculture, which allows the dense populations that make kingdoms necessary.