Even though other reviewers of this before me already explained what this show is all about, I'll attempt to do the same myself, but in my own way and differently. It originated as a Playstation video game, which led to the manga, which led to the anime. It's essentially about some high school students who lead secretive, double lives as agents for an organization that works to protect citizens from what are known as Invaders, which can be described as aliens in earthlings' clothing. These aliens take control over or possess earthlings and the agents, known as the titular Gate Keepers, have their own, individual special powers and attacks, which they use to create and open up these extraordinary gates to entrap, and destroy the Invaders. Interestingly enough, the show's setting, which is in 1969 and 1970, is fitting, since coincidentally, the '70s were the decade when the mecha anime, as well as the super robot and real robot anime would begin to take off, and mechanics is one of the cross-genres of this show.
In addition to this, this series contains an array of subject matters, such as, in one episode, the controversy of an arranged marriage. Many of the characters are great. I think the favorite for me would be Reiko Asagiri. I love the episode that centered on her in which she has a hard time trying to find out just how she fits in with the team and how she contributes. But later, she proves she is a useful member after all. That's just one of the positive messages contained in the series. But again, many of the others are fine. Another reason why I consider this to be worth watching is that it's one of those shows that have enough good moments to keep one involved or engaged. Despite this being among the anime that feature characters with large eyes that some may have issues with, the characters are still well-designed in every other way, at least. The backgrounds and layouts are well-designed, too. This is mainly 2D animated, but there's also some use of CGI, as is with some of the robots and vehicles used, which seem fine. The music is alright, but keeping the original, Japanese-sung theme song as is in the English dub is a nice touch. Speaking of the English dub, the voice acting in this sounds well done enough. I haven't heard the original, Japanese dub yet, but I plan to do so eventually. Aside from mecha, there's also comedy, drama, sci-fi, adventure, and harem, so since there's something for everybody, it will appeal to anybody who likes any of those. One of the reasons I decided to check out this, is because I'm trying to get into more shows featuring robots, that's the case for me. It's enjoyable enough for me, for the most part. However, I, too, wish that it ran longer and think it should have. I wouldn't have been ready for it to come to an end just yet, I really would've been happier and more satisfied if it had hit the syndication level of running. For one, I would've liked to have seen Baruko Ogawa (,a girl who for most of her appearances, was a recurring gag for her Marilyn Maneuver moments,) in action more, when it's later revealed that she, too, is an A.E.G.I.S. agent and I thought the viewers would've got to see her involved in that more, so all of that came as a surprise.
Another thing I meant to mention is one of the differences between the anime and the video game. A character named Francine Allumange, an agent from the French-branch of A.E.G.I.S., is a video game-exclusive character. Anybody who is a fan of her may be disappointed she's not featured in the anime, but there's still plenty else to compensate for that. I'll recommend this, nonetheless, because it's full of so much fun.
