The answers that you are looking for can be found beginning on p. 39 of Guns, Germs, and Steel. This is where Diamond starts to discuss the Great Leap Forward.
Your first question had to do with what this leap actually was. Diamond answers this on p. 39, though he does not define the leap in one single sentence. As you can see on p. 39, Diamond says that humans before the leap lived very simple lives. This changed with the leap. After the leap, humans started to have more sophisticated stone tools, tools made of things other than stone, multi-piece weapons, sewn clothing, houses, jewelry, and even art. The Great Leap Forward was the change in humans that allowed them to become so much more sophisticated.
I will address your third question next, since this is how Diamond does it. On p. 40, he speculates about what led to this leap. He is not certain, but he thinks it was an anatomical change. He says that he has argued in the past that the leap was caused by
the perfection of the voice box [which was] the anatomical basis of modern language, on which the exercise of human creativity is so dependent.
He says that other people think it was brain development that led to language. In either case, it appears to be the development of modern language that catalyzed the Great Leap Forward.
Finally, you ask which people were affected. Diamond poses this question on p. 40. He asks whether one group of people experienced the leap and then moved out around the world and spread their culture or whether people everywhere experienced parallel changes. Over the rest of p. 40 (and a little bit of p. 41) he says that we still do not know the answer to this question.
Diamond’s answers to the questions you pose are not stated very explicitly in the text, but you can find them beginning on p. 39.