The plot is quite simple, and supposedly based on a series of French novels (of which there are 97!) called "La Compagnie des glaces," or "The Ice Company." It follows Gainer Sanga, a skilled teenage gamer, who is one day dragged out of his high school classroom and arrested on suspicion of collaborating in an Exodus - something he'd never even dream of doing. He escapes with the help of a man named Gain Bijou and follows him for a while, before discovering Gain is an Exodus specialist who's been hired by some of the populace to coordinate one out of the Domepolis (a domed city) Gainer lives in, and before Gainer knows it he's stolen an antique Overman mech, assisted in kidnapping a princess, and been pulled into the Exodus scheme.
This involves a line of enormous City Units that are pulled by smaller tugboat-like vehicles called Silhouette Mammoths. Their destination: their ancestor's homeland of Yapan, where they hope to live away from the control of the Siberian Railway corporation which holds significant power in the Domepolis, it being the only source of new supplies. The main sources of conflict for them are the SR's attempts to stop the Exodus, as people leaving the Domepolis means less tax revenue and sales from the goods they transport (Tomino's not being very subtle here), and the London IMA organization, which owns the Domepolis and seeks to capture Gain for reasons concerning his past.
If some of this terminology is confusing, that's because it only really starts to makes sense in context. What's more complicated than the plot in King Gainer, as in G-Reco, is the setting. There's a small description in the OP but it doesn't tell us any specifics of the Domepolis our story begins in, Wulgusk, and neither do any characters for the most part. Exposition is sparse, and important character and worldbuilding details are given away in casual conversation then rarely repeated. You'll have to pay attention to pick up on all the details, but the setting here also isn't really as important to understanding everything as it is in G-Reco nor is it anywhere near as complex.
Making Gainer an individual who opposes the Exodus initially is a good move, though, because not only does it set up his character arc and allow the audience to learn about the details behind the Exodus, including the type of people who advocate it, it also creates an entertaining dynamic for interactions. King Gainer is filled with colourful personalities and fun interactions from and between all factions involved. Gainer is surprised to find out that both his teacher and the girl he crushes on in his class, Sara, are involved in the Exodus. The kidnapped princess Ana Medaiyu, a self-proclaimed "pitiable hostage," is more enthusiastic about the situation than anyone else. She and the rest of the people on the Exodus are developed surprisingly well both as individuals and as a group with a common identity throughout the show, and this is partially thanks to episodes that revolve around their antics: a supply raid on a lone Siberian Railway train, or a silly relay race to name a couple of examples.
While the point about corporations exerting control over people's lives is hammered in quite heavily, especially early on, the lighthearted tone (best exemplified in the series' catchy opening and claymation eyecatches) overall ensures that fun is prioritized over any kind of dull thematic lectures, and this is a positive - the representatives of the Siberian Railway are more bumbling fools than angry, vengeful capitalists. Even when the Exodus is forced to take in a large group of refugees they don't really have the supplies to accommodate, the unwavering sense of optimism that permeates the series seems to brush over it and insist that everything will be alright.
For TV anime standards the animation is fantastic, allowing for mecha fight choreographies that are just as interesting as the eccentric designs of the Overmen themselves - worthy of special mention is episode 14, handled by studio Bones and featuring simply jawdropping animation by the legendary Yutaka Nakamura. There's plenty of fast movement in scenes, and what's just as impressive as the mechanical animation is the detail put into animating the ice and snow breaking apart when a mech is hurled into the ground of the frozen tundra. You likely wouldn't think there are many opportunities to make use of the Siberian wasteland environment in fights, but this anime proves you wrong as early as the third episode.
Part of this is due to the creative abilities, or Overskills, the Overmen possess. King Gainer's first major opponent is an Overman that can stop time, and yes, this does lead to some scenes that feel heavily inspired by DIO from Stardust Crusaders (slightly unrelated but it's also amusing that there's a character voiced by Takehito Koyasu who more than slightly resembles Phantom Blood Dio). That's just the jumping off point for a range of unique and sometimes ridiculous Overskills. An episode featuring an Overman whose Overskill allows people to hear each other's thoughts produces one of the best episodes of anime I have ever seen. The titular mech has a gun that's also a chainsaw!
Their designs are contrasted by the grunt-type units the rest of the characters use, the Silhouette Machines, which are less explicitly humanoid and more militaristic in design, though not without their quirks. Gain's sniping unit, the Gotchko, has one thin skeletal arm and one muscular beefy arm. Turn A Gundam's the closest reference point I can think of here, and indeed the mechanical designer for this, Yasuda Akira (who would later also design for Code Geass and G-Reco), designed Turn A's characters, so there may be some inspiration from Syd Mead's mecha designs on that production involved here.
As far as the actual art itself, particularly for the characters, it's rather standard; as are the backgrounds except when they're depicting the awe-inspiring City Units - granted there isn't much you can do given the setting. More detail likely would have hampered the animation quality, so it's a fair tradeoff, but the predominantly flat colours and shading are unlikely to impress most, and in this area at least it's the weakest looking anime of those that feature Kenichi Yoshida's always fantastic character designs (it should be noted that there is a slight but noticeable improvement towards the end). While watching I couldn't help but think how gorgeous it would be if it were animated in the same art style as G-Reco.
The soundtrack is just as bold and dramatic as the action it scores demands. I'm especially a fan of the track "in close combat" with its guitar riffs and trumpets.
The meaning of the word Overman is put to the test in the Devil Gu- I mean, Overdevil arc, comprising the last seven episodes and introducing just the kind of potentially-world-ending stakes perfect for capping off the series memorably - the ending is especially good. It also brings some classic Tomino themes with it in case you thought they were strangely absent.
In the end Overman King Gainer falls just short of Tomino's biggest achievements, but this is no fault of the anime itself - it achieves what it sets out to do in providing a thoroughly enjoyable experience with great mecha action and endearing characters.
