Premise before starting: read Shonan Junai Gumi before starting to read GTO, or I will come to your house with a knife, many things of GTO, including the ending, are less powerful without having read Shonan Junai Gumi.

Having said that, however, GTO is a manga that if it is read in the right period (for example adolescence) can really change your life.
Obviously this manga has many logical flaws, but they are overshadowed by the fantastic character who is Onizuka Eikichi and his life lessons.
This manga raises several criticisms of Japan and its school system, which, according to the mangaka Tooru Fujisawa, produces water from all sides.
The genius of this manga lies in the fact that Fujisawa has put in this system made of sweet face and bad intentions, a pure and sincere character like Onizuka who does not think about money or the role but his students, also thanks to him, it becomes possible for Fujisawa bring out the rottenness of this system.

(sorry for the ingrish)