Bad Company is a single-volume prequel to GTO's predecessor, Shonan Junai Gumi. It depicts Eikichi Onizuka's life prior to his joining of the Oni-Baku. The manga was published in 1997 by Kodansha while Tohru Fujisawa was still working on GTO.
Shonan Junai Gumi (English: Shonan Pure Love Gang) is a 31-volume prequel to GTO. It depicts Eikichi Onizuka and Ryuji Danma as teenagers in the infamous Oni-Baku bōsōzoku gang. The shōnen manga was originally published between March 1990 and September 1996 in 31 volumes. In 2009, it was retitled GTO: The Early Years for the English digital deluxe release.
Ino-Head Gargoyle is a five-volume spin-off of GTO. It depicts Toshiyuki Saejima as the guardian of an ex-high-class escort named Arimori Shizuka. The manga was published between January 2012 and January 2014.
GT-R: Great Transporter Ryuji is a single-volume spin-off of GTO. It depicts Ryuji Danma and a girl named Mimon Ishikawa and their misadventures. The manga was published between June 2012 and October 2012.