''Gate Keepers'' started out as a TurnBasedStrategy (with DatingSim 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 Creator/{{GONZO}} who decided to omit [[PutOnABus 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, ''Anime/{{Gate Keepers 21}}'', set [[NonIndicativeTitle 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 ThePowerOfFive

----
!!This anime contains examples of:
* ActionGirl: Ruriko, Kaoru, Yukino, Feiling. [[spoiler: And the "Ooh, intense!" girl.]]
* TheAce: Jim Skylark.
* AllAmazonsWantHercules: [[spoiler: Kaoru and Bancho, after he gets his own Gate ability [[BirdsOfAFeather which is very similar to hers]].]]
* AllYourBaseAreBelongToUs: [[spoiler: With the exception of the Commander, his secretary, and the BridgeBunnies, everybody at AEGIS HQ becomes an invader]].
* AlternateHistory: 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...[[spoiler: ''[[HopelessWar much worse.]]'']]
* AnIcePerson: Yukino
* BaitAndSwitchCredits: 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.
* BarrierWarrior: Misao, in the games. Her {{expy}} Megumi, in the anime.
* BeachEpisode: Which is actually sort of plot relevant
* BerserkButton: One surefire way to piss Ruriko off is to call her a 'sniveler' (or just refer to her childhood being a sniveler). Though Shun does use it as an advantage at times.
* TheBigGuy: Bancho
* BlowYouAway: Shun's power, as well as that of [[LamarckWasRight his daughter Ayane]], [[spoiler:and his father]].
* BodyHorror: Two different types, depending on the series. In the case of ''GateKeepers'', the Invaders were sleeper agents, some sleeping so deep that they were horrified by the realization of what they really were. In ''Anime/GateKeepers21'', the Invaders are TheVirus.
* BoisterousBruiser: Bancho.
* BrainInAJar: Count Akuma's brain is in, for lack of a better word, a fishbowl.
* BrainwashedAndCrazy: [[spoiler:Ruriko]] in the final episode.
* CallingYourAttacks
* TheCavalry: Jim and his team in the final episode.
* ChekhovsGunman: That woman wearing the helmet and goggles [[PantyShot whose skirt always gets blown up]] is actually [[spoiler:an AEGIS undercover agent]].
* ChildhoodFriendRomance: Ruriko and Shun.
* ChineseGirl: Feng Fei Ling (as if that's not obvious enough). Even her invocation was in mandarin.
* CoolShades: The AEGIS Commander [[spoiler: as well as Reiji, when ''he'' becomes the new AEGIS Commander]]
* CrazyPrepared: AEGIS just happens to have a giant piano for the Gate Robot in case Reiko decides to pilot it instead of Shun.
* CuteBruiser: Kaoru
* {{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."
* DisappearedDad: Shun's father, [[spoiler:who died while testing a prototype Gate Engine.]] Also, Shun himself in ''Gate Keepers 21'', [[spoiler:who is revealed, in the novel, to have died fighting the Invaders.]]
* TheDitz: Reiko, though this might be out of trauma after her parents's divorce.
* TheDragon: Kaiser Kikai and Count Akuma.
* DrivenByEnvy: Ruriko is popular, beautiful, rich and kind. Megumi is socially-inept, plain-looking, middle-low class and embittered. [[spoiler: Guess which one pulls a FaceHeelTurn?]]
* EveryoneCallsHimBarkeep: The Commander. His name is revealed in the [[AllThereInTheManual novel]].
* EvilCounterpart: "Shadow"
* EvilMakeover: Happens to [[spoiler:Kageyama]] whenever he switches to his "Shadow" identity. Inverse Gates are pure black, regardless of their original color.
* {{Expy}}: Megumi is a strange case in that she coexists with the person she is expying, Misao Sakimori.
* FaceHeelTurn: [[spoiler:Megumi Kurogane, due to her ''massive'' case of being DrivenByEnvy, specifically her envy towards Ruriko.]]
* FunWithAcronyms: '''A'''lien '''E'''xterminating '''G'''lobal '''I'''ntercept '''S'''ystem.
* GenkiGirl: 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.
* GratuitousEnglish: Any American character who isn't part of Jim's team. Also Francine from the game.
* HealingHands: Ruriko
* TheHeartless: The Invaders
* HeelFaceTurn: [[spoiler:Reiji Kageyama, the BigBad of the original series, is the head of A.E.G.I.S. in ''Anime/GateKeepers21''.]]
* HeyItsThatVoice: There's quite a few recognizable seiyuu, especially Takahiro Sakurai and Ayako Kawasumi in some of their earliest major roles.
** In the English dub, there are [[LightNovel/HaruhiSuzumiya Wendee Lee]], [[Anime/CodeGeass Johnny Yong]] [[PlayingAgainstType Bosch]], and [[Anime/TengenToppaGurrenLagann Michelle Ruff]], among other prolific VAs.
* HopelessWar: [[spoiler:As explained in ''Anime/{{Gatekeepers 21}},'' the invasion is proceeding nicely (over half of humanity has been subverted at that point), there's no way to actually win, and the very power used to fight the invaders will [[EquivalentExchange kill the users. Or worse.]] Ouch.]]
** [[spoiler:The ending, however, has the tables turn in humanity's favor, with most of the Invaders wiped out. Granted it required a ResetButton, and that the remaining ones are still in the open, but still...]]
* HowDoIShotWeb: Shun struggles in every episode to learn how to use his ability properly.
* HumongousMecha: Five of them, actually. [[SchizoTech In 1969]].
* InstantFanClub: Reiji has at least three girls as members of an actual club that he started.
* {{Invocation}}: "Gate! Open!"
* JerkAss: 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 JerkAss shortly before the episode.
** Also Megumi. While not an invader [[spoiler: she does pull a FaceHeelTurn later on.]]
* LadyOfWar: Ruriko, who uses arrows, as opposed to Kaoru, who uses her [[CuteBruiser fists]].
* LamarckWasRight: Shun's daughter Ayane inherits her gate power from him. Furthermore [[spoiler: Shun himself got his power from his father]].
* {{Leitmotif}}: In the game each of the main female characters and some of the supporting characters have their own theme.
* LotusEaterMachine
* MagicMusic: Reiko's Gate power is focused on her piano playing, which creates beautiful illusions for people... and acts as a BrownNote for invaders.
* MasterOfIllusion: Count Akuma, and to a degree Reiko (see above).
* MeaningfulName: The Commander's name has something to do with his ability to manipulate time.
* {{Meganekko}}: Megumi Kurogane and Misao Sakimori
* MultinationalTeam: Jim Skylark's elite group
* MusicalAssassin: Reiko can use her powers when she's playing music, so...
* NewEraSpeech: [[spoiler:Once Kageyama takes over and declares himself Prime Minister]]
* NewTransferStudent: Reiji Kageyama, and Satoka Tachikawa, from ''[[Anime/GateKeepers21 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.
* TheOmniscientCouncilOfVagueness: Comes as either the invader leaders, or the group of AEGIS commanders
* PantyShot: 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 Maneuver}}s, more is visible each time, until we see a close-up of the front followed by her winking at the camera.
* PaperThinDisguise: 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.
* ParentalAbandonment: 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.
* PlayingWithFire: Feiling
* PointlessBandAid: Shun
* PsychoticSmirk: [[spoiler:Kageyama]], and [[spoiler:Megumi, after her FaceHeelTurn]].
* PutOnABus (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.
* ReallySevenHundredYearsOld: Yukino
* RedHerring: The old sweet potato vendor from one episode, who owns an exact duplicate of the Invaders' SinisterShades. It turns out that the real invader was [[spoiler:a cute white spitz that seemed irrelevant at the time. TheReveal was quite [[{{Narm}} narmy]], considering it showed the dog wearing sunglasses.]]
* TheRival: Jim Skylark, who is also TheAce
* SecretLegacy: Shun learns why his neglectful father was never home.
* SexySecretary: Who also happens to be SchoolNurse.
* ShockAndAwe: Jun [[MeaningfulName Thunders]], a member of Jim's team.
* ShoutOut: [[spoiler: The spitz in episode 18's]] transformation sequence is similar to Series/{{Ultraman}}'s
* SinisterShades: The Invaders wear these, as do the BigBad.
* SmallGirlBigGun: [[spoiler: The "Ooh! Intense!" girl turns out to be an undercover AEGIS agent who carries a {{BFG}}.]]
* TheSmartGuy: Megane
* SpiderSense: Misao Sakimori from Jim's team, who dictates the exact time before something happens, giving the group a serious tactical advantage. [[spoiler:Kageyama had the Gate of Foresight before his StartOfDarkness turned it into the Gate of Shadows. He got it back after he pulled a HeelFaceTurn.]]
* SpoilerOpening: The [[TheManBehindTheMan surprise]] BigBad gets revealed even before the first episode.
* StrangeSalute: The AEGIS Salute
* StudentCouncilPresident: Ruriko
* SuperCellReception: Many characters use specially designed cell phones to activate "Gates", the source of the series' [[FunctionalMagic magic-like abilities]].
* SuperPrototype: Gate Robot Unit 00, which has no PowerLimiter, unlike the later production models.
* SuperSpeed: Jim Skylark's Supersonic Gate.
* SuperStrength: Kaoru, who additionally has SuperSpeed, as her Gate power amplifies her physical abilities. [[spoiler: Also, Bancho in the final epusode.]]
* {{Technopath}}: Kaiser Kikai
* TelevisionTieInNovel: ''Gatekeepers 1985'', which bridges the gap between ''GateKeepers'' and ''[[Anime/GateKeepers21 GK21]].''
* TheAnimeOfTheGame
* ThoseWackyNazis: Kaiser Kikai has the look, and has a tendency to shout "HEIL VADER!" whenever he shows up.
* TimeStandsStill: The Commander's power. [[AllThereInTheManual Which is only depicted in the novel]].
* TransformationTrinket: Infected people somehow get SinisterShades from nowhere, which they put on to transform into Invaders.
* UnstoppableRage: Gatekeepers who go into an UnstoppableRage gain "[[ReversePolarity Inverse Gates]]", which do the exact opposite of their original powers. This is deconstructed in ''[[Anime/GateKeepers21 GK21]]'' [[spoiler:when the BigBad, whose normal Gate power is to erase things, gets pissed off when Ayane destroys her SoulJar. The last thing she does before fading away is use her Inverse Gate, which causes [[ResetButton 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.]]
* TheVirus: The Invaders in ''Anime/GateKeepers21''.
* WaifFu: Kaoru Konoe and her tank-lifting introduction.
* TheWorfEffect: Kaiser Kikai easily defeats Jim's elite MultinationalTeam with his [[MilitaryMashupMachine tank on a battleship]], forcing AEGIS to deploy the SuperPrototype.
* TheWarRoom: Any AEGIS base. AEGIS Japan comes complete with BridgeBunnies.
* ZettaiRyouiki: Feiling wears a long red qipao with slits long enough to creat the effect.
----