The anime makes reference to Onizuka and Ryuuji’s background as the Onibaku, but the GTO manga is actually a sequel to another manga of Tooru Fujisawa’s, Shounan Junai Gumi, when they were in high school. Though that series got an OVA series in the 90s, it has no connection whatsoever to the GTO anime, but some characters from that series also appear both in the GTO anime and manga, such as the Hakosuka Queen. In the GTO anime, you also hear of Ryuuji’s girlfriend (who has limited screen time in the anime due to studying abroad), and you do get appearances from Saejima, Onizuka’s corrupt cop friend (who gets his own dedicated chapters in the GTO manga).

If you want to know how Ryuuji and Onizuka first meet Saejima, and learn the story behind Ryuuji’s girlfriend, you have to read Shounan Junai Gumi to order to do so (and if you want to know how Ryuuji and Onizuka became friends, read Bad Company). Furthermore, you see how Onizuka in the GTO manga changes from his pompadour style in Shounan Junai Gumi, to the hair he has in GTO. Regardless, having to read Shounan Junai Gumi isn’t a prerequisite to enjoy GTO in both media forms but it does add more to the experience, and you’ll have a more contextual understanding of Onizuka’s relationships with Ryuuji and Saejima.

In both the anime and manga versions to GTO, Onizuka wants to be a high school teacher just so he could hook up with high school girls (but immediately finds the value of his work). However, the anime entirely leaves out the backstory to the reason behind those motivations. In the beginning of the manga, Onizuka is struggling to find a job and he runs into a high school girl who is having trouble with her boyfriend. After learning that her boyfriend was her teacher (a bald, short, middle aged man), it gave him the inspiration to become a teacher. In the anime, this story is cut out and it’s just simply because he’s a pervert that can’t get laid.

In the series, Tomoko, one of Onizuka’s students who tends to be clumsy (nicknamed Toroko, based on the word toroi, meaning clumsy), enters a pageant and even competes against her best friend, Miyabi. As part of the pageant, contestants have to give a speech. In both versions, Tomoko talks about how much her friendship with Miyabi means so much to her, but is executed in very distinguishing ways. In the anime, she is given a cell phone on stage and calls Miyabi’s house by leaving a message on Miyabi’s answering machine thanking her for being her friend. In the manga, the execution of the speech is presented rather comically but still conveys the same emotional content. In the manga, Onizuka throws her some dinosaur toys and with those toys, she re-enacts how she became friends with Miyabi. And speaking of Tomoko, she does get her own side story in the manga as she tries to become an idol.

Throughout the series, Miyabi is the ring leader over her class becoming a bunch of punks who drive their previous homeroom teachers to quit or go missing. The motivations in both versions significantly differ, but still relate to a former teacher. In the anime, Miyabi had a friend who got pregnant by a former teacher of theirs and when he refused to take responsibility, she killed herself. In the manga, her motivations aren’t as dark but in her younger years, she had a crush on a teacher and felt betrayed when she discovered he was already engaged. Shortly after, she takes explicit pictures of herself by saying her teacher did it, thus getting him fired (but at the cost of another friend getting expelled).