Be 'careful', though. Out of these four, only Gunnm isn't specifically targetted toward boys/teenagers (especially the second part 'Last Order').
GTO I guess you could get a laugh out of, and it can be a bit of interesting in what you learn about the Japanese school system.
Honestly for an adult who wasn't into manga until now, I think a much better choice would be:
- Berserk (Dark Heroic Fantasy)
- Blade of the Immortal (similar mood, though not as dark and in medieval Japan)
- Monster (Contemporary, mistery)
- MPD Psycho (Contemporary. Psychological thriller with a touch of SF)
- Nausicaa (Post Apo). As far as I'm aware, the only significant manga from Miyasaki. Don't be confused: manga=Japanese comics
. Most of Miyasaki's work is animation.
There are quite a few others, but this is a good start I think.