Summary Gatekeepers 21 is set nearly 30 after the original GateKeepers series. It is centered around Ayane, daughter of Shun Ukiya. Aegis no longer exists, except for a small group of people who secretly fight the invaders. WIll the new GateKeepers be able to stop the invaders before the world becomes full of corruption?
Audio 
I am really amazed at how good the music for this show is. The music really fits whats happening and I enjoyed it very much. Starting with episode 4, the opening and ending switch places, and you get a different opening animation (with the ending song from the previous 3 being the opening, and the opening from the previous 3 being the ending). And like the original series, it has a great dub.
Visuals
Animation is excellent. Easily some of the best I've seen in an OVA. Great looking character designs.
Story/Entertainment
Gatekeepers 21 is much darker than the original series. The size of the Gatekeeper team is much smaller, which allows for more focus on the few that are in the show. Ayane is a girl who hates all the trash in the world and wishes it would all go away. She dislikes her father and for this reason doesn't like to use her gate of wind, so she uses "false" gates instead, through the use of cell phones. Two other gatekeepers are introduced too. I really enjoyed the new characters.
Thirty years have passed since the first battle against the invaders. Much is left unexplained, but there are some cool scenes in here that you need to have seen the TV series to understand. Also, one of my favorite gateKeepers from the original series, Yukino, returns in this series for another fight against the invaders. I really enjoyed these scenes and fans from the original series will no doubt have as much fun watching them as I did.
GateKeepers 21 is a marvelous show. It's even better than the original. Even though it only spans six episodes, it gets through a lot, develops great characters, provided some good action and never felt rushed. Great stuff.