Guns, Germs, and Steel is essentially pop science by a guy with next to no expertise in many of the areas he bases his theories on like geography.

Germs - Ignores the fact that Europeans were devastated by things like Malaria that many Africans have moderate immunity to thanks to the sickle cell trait

Domesticated animals - Ignores that Europeans have actually used Zebras as work animals and had to domesticate cows from Aurrochs and dogs from wolves

The whole thing is basically Diamond starting off with a conclusion and finding a theory to fit in