Great Teacher Onizuka is a manga which was written and drawn by Tooru Fujisawa and it is his best-known work. There are 25 volumes of the manga which ran from 1997 to 2002. It is a highly popular series and its drama series featuring Takashi Sorimachi also gained recognition. Due to GTO’s popularity, Tooru Fujisawa also illustrated GTO Shonan 14 Days (2009-2011) and GTO: Paradise Lost (2014-ongoing).
I think the manga is funnier and more entertaining than the anime. The anime tones down a lot of things to make it more suitable for general viewers. The anime can also be quite slow-paced so that some of the funny parts in the manga end up not being funny in the anime. The manga has quite a lot of words, though, which requires you to have some interest in reading. Nevertheless, the manga is really a great read which would encourage you to read the barrage of words which may appear at times!
Even though the manga mainly revolves around the lives of teachers and high school students, it is suitable to be read by adults as well. Most of the students in the story may be too young which adults may not be able to relate to but the humor and content of the manga can be quite mature. In fact, I would not really recommend the manga to kids due to its sexual and violence content. The anime would be more suitable for younger audiences.
I highly recommend reading the GTO manga as it is among the best manga series I have ever read! I have reread it a few times and yet I still find it extremely hilarious. The illustrations are also top-notch. It is one of the classic series which every manga fan must read!