How can I write an introduction to Guns, Germs and Steel?
If I were writing an introduction, I would want to give my readers a basic idea as to what the book is about. So that is what I think you should do -- you should figure out how to summarize the main argument of the book in whatever space you are supposed to fill.
For me, the major point of the book is that some societies became stronger than others by the late 1400s because they were lucky. Their luck was geographical. In other words, some societies were lucky enough to spring up in places that were well located. These societies got to develop agriculture first and that led to them being able to have guns, germs and steel.
So I would start by restating Yali's question. Then I would say that Diamond thinks that most common explanations say that the strong societies got that way for cultural reasons -- because they had "better" cultures. But Diamond believes that it was all geographical luck.
I hope that helps.