According to Weekly Shonen Magazine, Fujisawa Tohru will start a new series of manga “Great Teacher Onizuka (GTO)” from issue 28. The title is “GTO Shonan 14 Days” and the story takes place in Shonan bay area.
Source: Scan of WSM
Update
According to Mainichi Online, the new series is not a sequel, but a new episode within the previous series.
Source: Mainichi