Yep, with changes because he was burned out and didn't want to finish the original storyboard he created for Paradise Lost.
This explains why I was misunderstanding some of the story's contexts. In the end, the prison part was sped up, and instead of the events taking place "during Onizuka's story" in the same original prison (the school parts were sort of "past" in the story and were told during the course), I ended up creating "another prison" in the end.
Anyway, I liked the ending, despite the stumble. GTO is sort of the story of a guy trying to survive in a world that has its flaws. And dealing with the underworld of the media, a prison full of problems, and on top of that, having a cult leader who's crazy about killing you while you're suffering; it's almost like a soap opera.
And from what I know, the author was already tired of Onizuka. And he's right, he gave a reasonable ending, OK. It's better for him to rest after a story that at least made us understand that we rebel against the world, but we learn from those who learned from their own rebellion to change the world.
(And of course, give a satisfying "piledriving" to a fascist along the way.)