The GLF caused loss of millions of lives, and aggravated Mao’s mistrust in Liu Shaoqi and Peng Xiaoping. Criticize against Mao from the military, such as Marshal Peng Dehuai, further scared Mao of losing controlling over the Party. He stepped down for a while and then he fought back in 1966. Thus, GLF was the prologue of the Cultural Revolution.
Here in my answer, I wanna focus on the number of deaths in GLF which has been very controversial. My estimation for it is between 5 million to 15 million. 30 or 40 million are very unlikely, since the population in China was 640 million in 1957, a population loss of 5%-6% was too conspicuous to hide.
Believe it or not, I did my own investigation on it. Unlike others who just did some searching jobs on internet, I asked old people who experienced the period for first-hand information. I travelled a lot to more than 10 provinces of China in the need of my work in the last ten years, and by the way I got chances to interview dozens of old people in the rural.
I found that almost all people were hungry due to shortage of food caused by GLF, but deaths were highly heterogeneously distributed. Deaths by starvation were disastrous in Henai province and some regions of Sichuan province and Anhui province. The worst I heard was most people in a village in Henan province died and nobody could bury them in time. However, deaths of starvation were rare or fewer in other provinces such as Jiangsu, Hubei, Hebei.
Thus I concluded that in addition that Mao was the first one responsible, the act of the first secretary of a province who was responsible to implement the wrong policy played a very important role. If Mao’s requirement was 100, some provinces did 50 or 70 like Guangdong, Hubei, etc., whereas some provinces did 120 or even 200 such like Sichuan and Henan, where most fatalities came from.