It is a little bit difficult to say that Guns, Germs, and Steel has an outcome. A non-fiction book is not typically said to have an outcome. As it is, I suppose we could say that the outcome of this book is that Diamond says that he has proved his thesis. Of course, it would be a huge surprise if he did not prove it.
So, we can say that the outcome is that Diamond proves that geography is the main factor that determined which area of the world became dominant. The point of this book was to try to determine why Europeans came to dominate the world. Starting in the 1500s, it became clear that Europeans were the dominant people in the world. This continued up into the 20th century. The question, then, is why this happened.
The outcome of this book is that Diamond proves that geographic luck was the cause of European dominance. He shows that people in Eurasia had more access to plants and animals that could be domesticated. This allowed them to become civilized much earlier than other peoples. By becoming civilized earlier, they became more powerful. This happened not because they were somehow superior people, but merely because of geography.