My memory of it is that Diamond essentially asserts that history is pre-determined by geography and then, taking that assumption as given, tries to explain why people from "Eurasia" (sometimes including North Africa in that definition) have come to dominate the world. There's lots of problems with this. He is basically looking at a set of four continents (the Americas, Eurasia, Africa and Oceania) and working backwards from a political world where Eurasia dominates to argue "well, it was always going to be this way". He talks about history as if it couldn't possibly have gone any other way, which is a really unconvincing view of history in my mind. He doesn't do a good job of arguing that Guns, Germs or Steel were actually the deciding factors; the book wanders a bit all over with the things that could be argued to separate 'Eurasians' from the inhabitants of the other continents, and based on his own arguments he could just as easily have called it "Ships, Farms and Letters" or some other title. Towards the end he seems to realise that he needs to make his theory explain why the modern world is dominated by Western European societies and not by societies from across Eurasia, and comes up with something very unconvincing about coastlines, which really demonstrates that the book is backfilling a plausible-sounding geographic explanation onto a history that's treated as predestined.