Onizuka is back but in prison giving a recount of his story teaching the idol/celebrity class. The manga spends a fair bit of time covering Onizuka in prison. It will be back and forth between recount and real time experiences. A few of the previous students return, but are unfortunately side characters. The story seems to not be so much of a sequel but more so set in the present time. Onizuka, is still teaching in the same school, but a lot of things are different now. The students he teached are different (which makes sense), the school faculty is different (with a few returningback). To a certain extent, if you are expecting a continuation from GTO, the only constant you'll get is Onizuka's approach/mindset in resolving problems. He is just as hot-headed and impulsive as before, acting before thinking. And if it doesn't work out, he starts to contemplate of a plan B. The story plot also spends a significant amount of time, no longer focusing on students but on the teachers and their personal life outside of school.

Recommended:
1) If you enjoy recount-based story plots
2) You like prison-theme story plots
3) You like cult-theme story plots
4) You enjoy series like Sket Dance/Gintama, with a more touch-and-go storyline

Not recommended:
1) If you are interested in most of the past characters returning back
2) You like a continuous growth theme, where there is follow-up to the characters after the initial problem is dealt with