The episode 1 of Gate Keepers 21 was originally aired in 2002.
No. "Encounter"
Transcription: "Deai" (Japanese: ) April 24, 2002 (2002-04-24) 
April 24, 2002  January 8, 2003 The OVAs were periodically released from April 24, 2002 to January 8, 2003. 
