So... is Gate Keepers finished or not?
NO, it is not finished.
Why?
The anime finished after the manga:
Anime finished: January 08, 2003
Manga finished: October 31, 2001
... But MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
The mangas and the non-core contents are excluded by default, but you can enable them by clicking the buttons below:
Gate Keepers
ゲートキーパーズ
Anime - Base
Release date: April 03, 2000
Gate Keepers 21
ゲートキーパーズ21
Anime - Sequel
Release date: April 24, 2002
Synopsis
In an alternate 1969 Tokyo, a secret organization called AEGIS is humanity's only defense against mysterious beings known as Invaders.
These Invaders have infiltrated society, and only individuals with the power to open "Gates" can truly combat them.
AEGIS has a special team called the Gate Keepers, composed of people with unique Gate-related abilities.
The story centers on Shun Ukiya, an ordinary high schooler who unexpectedly discovers his own Gate-opening ability when Invaders threaten his family.
His newfound power draws him into the world of AEGIS and reunites him with a childhood friend, Ruriko Ikusawa, who is already a Gate Keeper.
Shun must now learn to control his abilities and work alongside the Gate Keepers to protect humanity from the encroaching Invader threat.