I'm a little sad that such a great manga doesn't have any reviews or people talking about it yet. GTO is definitely a blast from the past and a great reminder that some of the older classics still got it today.
GTO is the story of ex-biker gang leader Onizuka who decides to become a teacher to get snag a nice high school babe. However, as he gets assigned to his first real class, he finds out that his plan won't be that easy since he gets stuck with a class that has purposely made three of their previous teachers resign. Yet for some reason, Onizuka is able to reach the kids like no teacher ever had before, and slowly begins to solve their problems and leaves them off much happier than before. The story feels pretty slice of life since, while everything is in at least a mini arc, not too much passes along from one arc to another. The first third pretty much sets up the pace and the characters while the rest is spent on three longer arcs that further explore major characters and plot points. The major arcs feel a whole lot more intense than the rest of the story, which makes for a nice change of pace.
The characters are obviously going to be the main point of a manga like this. Every character has their own type of unique charm, with Onizuka being this super rough around the edges guy with a heart of gold, his coworker being a well-meaning but unsure teacher, and each student being their own little, lovable mess of problems. The best part about GTO is seeing these characters who were once burdened by all these problems change and grow to enjoy everything that's happening around them.
---Long winded discussion of themes below, skip if you're not interested---
And with all that basic stuff out of the way we can talk about some major themes in GTO. Now, there are definitely a few big ones like don't judge a book by its cover, but the one that I think GTO does the best job at showing is that life sucks but that doesn't need to matter. Almost every arc in the story revolves around someone who cannot handle the things they are going through and end up getting other people involved in the way they choose to deal with it. Some of the characters attempt to take it out on themselves with suicide, some push it onto others by pulling dangerous pranks, and some deny it all together or try to justify what they've done as being necessary. And in each one of these cases, Onizuka has to step in in one way or another to diffuse the situation.
The crazy thing about this manga is that it doesn't try to just focus on the good times or advocates to forget about the bad. Instead, it recognizes that the things that these kids went through were terrible, and that they did have every right to lash out for what has been done to them. It's not like you can just take things like ruining someone's professional and romantic life, severe bullying or rape and say that they didn't happen or that they don't matter anymore, because they do. Every wrong thing people do has a consequence and everything done to them leaves a mark, but that doesn't mean they have to stay there.
The scene that really sticks out to me the most is when Onizuka takes one of his kids on a joyride since they had been dealing with really tough family problems, and at the end of it all he brings here down to listen to a talk between the members of his gang where they all share about the terrible family experiences they all went through. Though they were all ruffians and weren't the type of people you would want to necessarily associate with, you couldn't say that they weren't happy with their lives now. They never once denied the fact that these terrible things happened, but they didn't let those things stop them from living their lives.
The only people you see in GTO that never get better are all the other teachers. They are legitimately pretty terrible people who face really tense situations in their personal lives. So they use their 'pastimes' like hardcore peeking up girls skirts to justify their actions. They are the only characters who never own up to their problems to face them head on, but rather choose to ignore them all together. GTO is very clear about the message it wants to send: life will bring many hardships, and it's okay to be upset and angry about them. But the one thing you can never do is to stay there with the problem and never face it. The only way to move forward is to accept the bad things that happen and look towards a brighter future. To experience the blessings you have now instead of wallowing in the things you can't change.
Overall GTO is a crude and crass manga with a whole lotta heart. It's something so unexpected from a manga like this, but I guess that's what they wanted you to think. It's not perfect and there are a few things it could have definitely handled differently, but I really believe that if the comedy doesn't get you, the soul of the manga will.