Definitely one of the best anime/manga out there is. This anime is a masterpiece in its own way with themes of masculinity, coming of age, redemption, challenging societal tensions and issues in an epic and obscure way. GTO is mostly humour and comedy and even some questionable explicit content which should not be taken seriously, it involves some heavy themes people deals with like bullying, depression, trying to find redemption, changing oneself etc.
In my opinion though kids should not watch this due to the themes I mentioned above - this manga/anime was released during the time when each and every and most of the mainstream manga was issued in the daily shounen manga so that's why it titles under shounen genre de facto. I would recommend being atleast 17+ to read and watch GTO if you will.
Now, the plot, the story, the overall character development, the characters and the themes of this anime is actually a masterpiece and I would recommend anyone who wants to read or watch a good seinen this anime. Characters are by far the most relatable in all of anime and the theme is extremely realistic and grounded.
Enjoy Reading.