There are also some trains of thought in GG&S that hew close to straight-up race science if you follow them to their logical conclusions. The oft-repeated "virgin soil" thesis is one of them. Diamond suggests that the biological conditions of the Americas made it so that Natives are less resilient to epidemic disease than Eurasians, and that this lead inexorably to mass pandemic death throughout the Americas as soon as a European touched down on the continent.
The problem is that this takes a real piece of history—that epidemics of newly introduced diseases tended to break out in areas experiencing colonization—and simultaneously extracts it from the material conditions of colonization and essentializes it to a racial characteristic of Native Americans.
First, regarding the myth of weak immunity as a racial characteristic: it isn't. It's true that epidemic diseases being introduced to populations with no historic exposure to them can be especially deadly, and that this is a contributing factor to high epidemic mortality in colonial environments. It does not follow that the Indigenous people of any region therefore have weaker immune systems or are particularly vulnerable to epidemic diseases generally. In fact, most of the diseases associated with epidemic outbreaks during the colonization of the Americas are ones where humans acquire immunity through exposure. After weathering one single smallpox outbreak, a population is no more vulnerable to smallpox than one that has had periodic outbreaks for all of recorded memory.
Additionally, human populations tend to recover quickly after major epidemics. The population of Europe was cut in half (and into smaller fractions in the places hardest hit) by the Black Death, but a hundred years later they had recovered form that population loss. Why could Indigenous populations not recover from similar losses? Even massive population declines could be attributed purely to epidemic disease, this would not explain the lack of a similar recovery.
Furthermore, many of these colonial diseases (e.g. yellow fever, malaria) were endemic to Africa but not Europe, meaning the Europeans would have been just as vulnerable to "virgin soil" epidemics as the Native Americas. And they were! During the first centuries of the colonization of the Americas, Europeans and Africans alike went through a several year period of "weathering" upon reaching the Americas during the which they were highly vulnerable to disease and had a very high mortality rate. Why, then, didn't these diseases wipe out colonial populations as they are claimed to have wiped out Indigenous populations? Why did they not flow back into Europe aboard so many treasure galleys and become a huge pandemic that cut the European population into a fraction of itself, as they are supposed to have done in the Americas?
This brings us to the second major problem: the material conditions of colonization. Man does not die by pathogen alone. Regions experiencing European colonization faced many consequent ills. European ships were often merchant ships, pirate ships, and slaver ships all in one, meaning any area in contact with Europeans experienced widespread and unpredictable violence and human trafficking. This violence spread far beyond areas of direct contact with Europeans, though, as European colonies often produced a high demand for enslaved labor, meaning that the Indigenous slaving industry would boom in regions affected by colonization.
Furthermore, in North America at least, it was a common practice to cope with population losses by warring with neighboring nations and adopting war captives into one's own society, exacerbating problems of endemic violence. In such a context of heightened warfare, raiding, and human trafficking, epidemics become extremely hard to combat. People are forced to live close together for safety, and people are moved between communities against their will. This produces what are essentially perfect conditions for the spread of disease, along with all of the other causes of mass death that accompany colonization.
TL;DR Diamond engages in myths about colonial history that essentialize the consequences of colonial violence to what is essentially (although he wouldn't put it this way) a biological racial characteristic. He paints the results of colonization (massive loss of Indigenous life and destabilization of Indigenous societies) as the causes of colonial social relations, rather than their effects.