Gate Keepers is a 24-episode anime (aired during the Spring/Summer 2000 season on WOWOW TV) based on the sci-fi console game of the same name. The general premise is a group of invaders (creatively called Invaders) begin terrorizing Japan during the late 1960's to early 1970's, and it's up to A.E.G.I.S. (a top-secret division of the SDF) to stop them, using the "Gate powers" found in particular children. It's a fairly generic plot, but as the story develops, the symbolism starts to seep through the typicality and the creators' message becomes obvious.
GK has developed a fairly large fan base in it's short life, primarily due to character designer Keiji Gotoh, who made Hoshino Ruri (from Kidou Senkan Nadesico) one of the most popular anime heroines ever. Most of the characters of Gate Keepers look like they and the Nadesico crew were separated at birth, and most people seem to think that's a good thing.
Spoilers ahoy.
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Another big aspect of it's popularity is how they depicted 1970's Japan. A lot of effort was put into making the scenery accurate, or at least believable (the underground base and the mecha can only be so accurate). For example, in the first episode, Ukiya (the primary male character) is shown flipping TV channels with a one-button remote, and everything about the scene is spot-on, down to the shows displayed on the television. The characters' attitudes also seem more like those of the average 1970's citizen, with little of the over-Westernization of modern Japan. (Okay, so I was told it was that way, as I was 2 years old on a different continent. Grain of salt, please.)
Along those same lines is the "real" plot, the above-mentioned symbolism. The 1970's in Japan is often referred to as "the Me decade" (like the US with the 80's), the beginning of the nouveau-riche and embracing of capitalism as it is now (popularly called "Me-ism"). Traditionally, the average Japanese thought in terms of "us," not "me" - the community always greater than the sum of each individual. This story is the re-telling of how it happened with a big sci-fi bent.
The Invaders represent, among other things, the greed and selfishness that overcame people during that era (much deeper than that, but watch the show yourself =P), and A.E.G.I.S. and the Gate Keepers represent traditional Japanese values, each Gate power reflecting a particular ideal. Then the show starts to explore all sides of the fight to show that no one side, not even the neutral side, is right or wrong, forcing the Japanese audience (who had recently seen their bubble economy pop) to contemplate the what if's and why's of the current state of the nation. Pretty amazing for a show about kids who get to skip school to beat up the bad guys.
(The spoilers should be over now, you can open your eyes.)
At face value, though, Gate Keepers is a sci-fi action show. Plenty of 3D CG, fight scenes, and comedy to satisfy most any anime fan's desires. It has a significant lag in the middle of the series, where it concentrates more on developing the "hidden" plot and the characters, so I wouldn't say every fan will like the show entirely. However, even without following the plot, the eye candy is more than enough to retain the viewer's attention (any Keiji Gotoh fan knows how much emphasis to put on the word "eye" there ^__^).
The animation was produced by Studio Gonzo, of Blue Sub no. 6 and Vandread fame (also of the Lunar game series). This was one of their first full projects (previously doing mostly only 3D animation and game/software development), and the one that proved they are one of the strongest forces in the anime industry. They were also involved in creating the Gate Keepers game. The November 2000 issue of Newtype (current as of this writing) rates GK at #5 in their "Ninki Sakuhin Best 20" (all-time top 20) listing, just under MS Gundam.
Though this show may not become too popular outside of Asia, due to it's very culturally-specific nature (like most anime), I would recommend this to anyone who enjoyed Evangelion or any of the Gundam titles. Both those shows base themselves around more locally-relatable topics (Christianity and war, respectively), but it's still worth more than a glance (face value in this case is worth quite a bit). I highly recommend it to any Nadesico fans, especially fans of the movie. It's currently not domestically licensed, and only available through fansubs.
Information taken from the show, Newtype, Animage, www.gonzo.co.jp (Studio Gonzo), and www.gate-keepers.com.
And try as I might, I don't think the Mozilla project will ever change their xmlns:xul directive from "gatekeeper/there.is.only.xul" to "xul/there.is.only.gate.keepers"... Saa ;_;