The answer to this question can be found a few pages into Chapter 13 of Guns, Germs, and Steel. It is on p. 242 of the paperback edition of the book.
In this book, Diamond is trying to answer “Yali’s question.” He is trying to determine why it is that Europeans came to be so dominant in the world. That is the central problem of the book in general.
In Chapter 13, Diamond is going to discuss technology and how it comes about. He wants to know why the inhabitants of some continents had so much more technology than the people of other continents. This is tied very closely to the central problem of the book since technology is one of the things that allowed Europe to dominate. Therefore, as Diamond says on p. 242,
In this chapter we shall finally come to grips with a central problem of this book: the question of why technology did evolve at such different rates on different continents.
In this chapter, Diamond will argue that geographic luck determines which continents evolved technology more rapidly and which do not.