In Guns, Germs, and Steel, Jared Diamond is not particularly interested in parts of the world that are not suitable for food production. For Diamond, places that are never going to be suitable for farming are not terribly important. Diamond does discuss such areas, for example, at the beginning of Chapter 5. There, Diamond tells us that there are “large areas of the globe” where “ecological reasons” make food production “difficult or impossible” even today. There are some places, like the Arctic or deserts, that are completely unsuitable for food production.”
However, these types of places are not particularly important to Diamond. He is much more concerned about areas that were or could have been suitable for food production but where food production did not actually develop. This issue is very important to Diamond because he says that areas that got food production first were the areas that ended up becoming world powers.
According to Diamond, the areas that were most suitable for food production in ancient times were the places that were geographically lucky because they had good native plants and animals. According to Diamond some places had many native plants and animals that could be domesticated and were therefore very suitable for food production. Conversely, there were some places that had few domesticable plant and animal species. Those were the places that were not suitable for food production.
So, there are at least two possible ways to answer this question. We could say that the places that are unsuitable for food production are places like deserts and arctic areas that were not suitable for food production long ago and are still generally unable to produce much food. However, the more useful answer is to say that, in Diamond’s view, places that did not have many domesticable plant and animal species are the ones that were not suitable for food production.