The book is about the patterns that drive human history, both the written history that we know and the vast period of prehistory which we have a pretty good idea about these days due to genetics and linguistics. A recurring pattern has been people moving around, displacing less fortunate people. Further, Diamond looks for root causes to explain why some tribes or nations would gain an edge over their neigbours, and a lot of the explanation is ultimately found in technology and animal farming.
Before I read "Guns, Germs, and Steel", I had been under the impression that the world was a fairly static and ecologically stable place until the European age of discovery started uprooting everything. In reality it was nothing like that. Almost every strip of land is inhabited be people whose ancestors fought off other people, and cultures expanding beyond ecological sustainability and suddenly collapsing is a common event.
Runner-up: anything by Hunter S Thompson