The book gives an account on historic events and some causes based on archeology, genetics research, linguistics and other fields, with a focus on the differences between Eurasia and the Americas + Australia. Those were as accurate as possible when the book was written, and are not substantially different in 2020, even if some data has changed.
However the main theory advertised by the book remains is based on environmental determinism, which is not an objective way to approach history.
Whenever comparing 2 groups of people that have diverged in any characteristics (looks, abilities, power, technology), there are common categories of factors that might explain the difference, environmental factors (geography, climate, terrain, resources, climate change), biological factors (available animals, plants, diseases), catastrophes (volcanoes, droughts, floods, pests), cultural factors (language, law, religion, politics, trade, economy), genetic factors, important personalities (Alexander the Great, Aristotle), inventions and discoveries, important battles, wars, outbreaks of epidemics.
Those are not even all, just examples. When historians create theories to explain causality of events, the safest bet is to say "It's a combination of factors, it's complicated".
However in GG&S, all difference are reduced to food production and geography as "ultimate causes", and when this is too difficult then the author allows for other causes as "proximate causes", still determined by "ultimate causes". That way, the differences of power of populations until 1500 CE all appear to be derived from the environment.
However, the logic is not viable. Just because we know that population P1 developed faster than population P2, and we know that P1 had an advantage X, this does not mean we can infer X alone determined how much faster P1 developed than P2. Even if we know that the "fertile Crescent" had advantages in wild plants and animals, this does not tell us whether the speed by which it developed compared to other regions was 100% determined by this advantage, or whether this improved the speed by 50% and other factors were equally important.
The theory of the book is nevertheless better than other simplified theories like:
- "Western Europe got most powerful because God chose the white race as his most beloved creation and led the white race to victory" (divine determinism)
- "Western Europe got most powerful because the white race is superior, genetically" (genetic determinism)
- "Western Europe got most powerful because Judeo-Christian philosophy is superior to others" (cultural determinism)
- "Western Europe got most powerful because it was lucky enough to have most geniuses in it's history" (chaotic random determinism)
But GG&S still falls into the same category of theories that try to reduce complex history to just one simple factor. While it seems obvious that any continent achieving food production earlier than others is an important factor in becoming dominant (especially between distant continents), this does not eliminate potential other factors also being involved. The differences between connected Western Europe, Eastern Europe, Western Asia, East Asia, and the different parts of Africa are not explained very convincingly by the theory provided in GG&S, the author merely defers this to "extended research".
Nor is there any other competing simple theory explaining the differences well within Afro-Eurasia. And the most honest and accurate theory may still be: "It's complicated".