For the most part, you are right about Diamond’s main thesis in Guns, Germs, and Steel. The one thing I would say is that you should not say that he is trying to “a country’s outcome.” Rather, I think that you should say something like “geographical location determines which parts of the world” had better outcomes. The reason for this is that Diamond is not trying to explain why, for example, France is more powerful than Spain. Instead, he is trying to explain why Europeans were more powerful than Native Americans, New Guineans, Australian Aborigines, and other groups. So, Diamond’s basic thesis is that it is geographical luck that determined which areas of the world became powerful.
To support this, he says that farming societies are more powerful than non-farming societies. He says that farming societies are able to develop the “guns, germs, and steel” that make them powerful. This means that the regions that developed farming first would become stronger in the long run. This is proven by the fact that farming developed in Eurasia and people from that land mass came to dominate the world.
So now the question is why farming developed where it did. Was it because people in one place were smarter or more determined or in some way better than other people? Diamond says it was geographical luck. As proof of this, he shows us that Eurasia had more wild plant species that could be domesticated (see p. 140) and more wild animal species that could be domesticated (p. 162). He says that farming started up not in the places where the people were smartest, but in the places where there were plants and animals that could be domesticated. This is a result of geographical luck. As one bit of proof for this, Diamond argues in Chapter 15 that the Europeans were not able to create agriculture in Australia using native plants and animals any more than the aborigines were. The Europeans had to bring in their own plants and animals (which they had because of geographic luck) before they could start food production in Australia. These are a few of the more important facts that Diamond cites to support his thesis.