In Guns, Germs and Steel why did Native American societies lag so far behind Eurasian societies (chapter 18)?
A careful re-reading of the chapter you have identified would help you to answer this question and also give you the background knowledge that the necessarily brief response I can supply you with would lack. However, just to give you a summary of the main points of this chapter, which will hopefully encourage you to revisit it and read it in its entirety.
The chapter identifies four key factors that were essential in the different rates of development between Native American societies and Eurasian societies, which of course gave Eurasian societies the "edge" when the two civilisations finally collided. Diamond offers the following helpful summary:
Thus, we have identified three sets of ultimate factors that tipped the advantage to European invaders of the Americas: Eurasia's long head start on human settlement; its more effective food production, resulting from greater availability of domesticable wild plants and especially of animals; and its less formidable geographic and ecological barriers to intracontinental diffusion. A fourth, more speculative ultimate factor is suggested by some puzzling non-inventions in the Americas: the non-inventions of writing an wheels in complex Andean societies... and wheels' confinement to toys and their eventual disappearance in Mesoamerica.
All of these factors meant that crucially the balance of power was tipped towards Eurasian societies. To gain a full understanding of each of these four ultimate factors, you will need to refer once again to the book to gain an understanding of how this factors led to dominance.