Gate Keepers started out as a Turn-Based Strategy (with Dating Sim elements thrown in) video game for the Playstation in 1999. A manga was also commisioned as well and both were released in the same month. It was later adapted into an anime series in 2000 by GONZO who decided to omit one of the main Gatekeepers and replace another with another character with a similar design. The setting takes place on Earth, after the end of World War II. In the middle of all that, alien creatures known simply as Invaders are infiltrating Earth and attempting to take over the world by turning humans into soulless robots. The only means to defeat the Invaders is by tapping into powerful energy locked inside dimensional "gates".That's where a secret agency known as A.E.G.I.S, short for "Alien Exterminating Global Intercept System," comes in. They seek out people known as Gate Keepers, who have the ability to unlock the gates, which in turn gives them superhuman powers. The protagonist, Shun Ukiya, is one of the Gate Keepers, who becomes aware of his powers after witnessing a fight between the Invaders and a schoolmate — and childhood friend — Ruriko Ikusawa. The story goes on to include other Gate Keepers and their exploits as they fight against the Invaders in their bid to defend Earth.It also spawned a sequel, Gate Keepers 21, set 31 years later. There is also a novel, set in 1985, which helps explain some of the events between the two anime series.If you were looking for the young adult series of novels about the Gatekeepers, see The Power of Five
This anime contains examples of:
Action Girl: Ruriko, Kaoru, Yukino, Feiling. And the "Ooh, intense!" girl.
Alternate History: It's mentioned that the Invaders were first encountered in 1945. The events that transpire in the show itself, as well as the existence of AEGIS, could also count. Though by 2001 (Anime/Gatekeepers21), history has more or less become similar to our own, if not worse...much worse.
Bait-and-Switch Credits: Subverted. One scene from the OP apparently depicts Megumi firing a ki blast from her hand (which is naturally impossible due to the nature of her powers, and thus, wouldn't happen). It turns out she was using her powers to block that energy blast, not to produce it.
Body Horror: Two different types, depending on the series. In the case of Gate Keepers, the Invaders were sleeper agents, some sleeping so deep that they were horrified by the realization of what they really were. In Gate Keepers 21, the Invaders are The Virus.
Delinquents: Shun and Ruriko disguise themselves as delinquents (Shun wearing his uniform in disarray, Ruriko dressing like a sukeban with a long skirt and curly hair) in order to confront Bancho — whose nickname itself is Japanese for "delinquent."
Disappeared Dad: Shun's father, who died while testing a prototype Gate Engine. Also, Shun himself in Gate Keepers 21, who is revealed, in the novel, to have died fighting the Invaders.
The Ditz: Reiko, though this might be out of trauma after her parents's divorce.
Genki Girl: Francine although she doesn't reach the heights that many of the other characters do for the trope and can be very serious when she wants to be but her goal in life is to make one thousand friends.
Gratuitous English: Any American character who isn't part of Jim's team. Also Francine from the game.
Jerk Ass: Most of the undercover Invaders are like this in their human disguise. One episode that centers around one shows that this may partly be due to stress as it was mentioned that the invader only started acting like a Jerk Ass shortly before the episode.
Also Megumi. While not an invader she does pull a Face-Heel Turn later on.
New Era Speech: Once Kageyama takes over and declares himself Prime Minister
New Transfer Student: Reiji Kageyama, and Satoka Tachikawa, from GK21. The gatekeepers, barring Ruriko, also apply, as they all had to transfer to AEGIS's combination school/secret base when they join the team.
Panty Shot: In the very last scene of the last episode: as Shun and Ruriko stand together watching a sunset, Ruriko's skirt blows in the wind and the camera twice gets a clear view of white panties.
Also the shtick of the "Ooh, intense!" girl. Over the course of no less than five Marilyn Maneuvers, more is visible each time, until we see a close-up of the front followed by her winking at the camera.
Paper-Thin Disguise: It's inexplicable how no one realized Kageyama was their enemy when all he puts on for disguise is a pair of shades (which he carries around everywhere anyway) and a little unbuttoning of his shirt.
Parental Abandonment: Shun's resentment of his dead father; Kaoru, Reiko, Megane, Big Boss and Feiling have no visible parents (Reiko's are divorced); Yukino's parents died some eight hundred years ago. Averted by Ruriko's intact and happy family. Possibly subverted by Megumi's apparent desire that her own parents would just go away and die somewhere.
Put on a Bus (Francine, who was one of the main Gatekeepers in the game, was completely omitted in the anime. Likewise Misao's role in the game was replaced with Megumi in the anime (it should be noted that the characters do have different personalities). Both Misao and Jun also played a more important role in the game and were both main characters as opposed to their brief cameos in the anime.
Red Herring: The old sweet potato vendor from one episode, who owns an exact duplicate of the Invaders' Sinister Shades. It turns out that the real invader was a cute white spitz that seemed irrelevant at the time. The Reveal was quite narmy, considering it showed the dog wearing sunglasses.
Spider-Sense: Misao Sakimori from Jim's team, who dictates the exact time before something happens, giving the group a serious tactical advantage. Kageyama had the Gate of Foresight before his Start of Darkness turned it into the Gate of Shadows. He got it back after he pulled a Heel-Face Turn.
Unstoppable Rage: Gatekeepers who go into an Unstoppable Rage gain "Inverse Gates", which do the exact opposite of their original powers. This is deconstructed in GK21when the Big Bad, whose normal Gate power is to erase things, gets pissed off when Ayane destroys her Soul Jar. The last thing she does before fading away is use her Inverse Gate, which causes everything to go back the way they were. Satoka points out the possibility that in her anger, she forgot that the Inverse Gate does the exact opposite of the regular gate.