Gate Keepers 21 is a six-part sequel OVA series to the Gate Keepers animated TV series and has a darker and more serious plot than its predecessor. Some of the questions left unanswered in the TV series are answered in this sequel. Linking the two series are the characters Reiji Kageyama, Yukino Houjo, and the former Far Eastern AEGIS Headquarters setting, Tategami High School.

Gate Keepers is a Tactical role-playing game developed by Entertainment Software Publishing and published by Kadokawa Shoten for the PlayStation. The game was then adapted into a manga series written by Hiroshi Yamaguchi and drawn by Keiji Gotoh and an anime series produced by Gonzo, and first aired on April 3, 2000. A six episode original video animation (OVA) sequel, Gate Keepers 21, was released in 2002. The main characters in the OVA sequel were the next generation of Gate Keepers.