Man. After just one reading, I know I'm unable to deliver any sort of proper review for this. But what the hell.
The story follows Yuki Tachibana, a very internalized, eccentric third grader, and his growing friendship with Makoto Suzuki, one of the few human characters in the story that does not ostracize Yuki and attempts to gain a kind of understanding with him. The two other key characters are Ganz, an elderly caretaker that recognizes Yuki's position -- where, unlike the others, he is not a skeptic of The Other Side that the boy describes -- and IQ, an acute fifth grader with a box over his head, allowing one peephole to see the world outside of himself. Yuki contends at different points in the book with various choices: whether the monsters he sees are real or hallucinatory, and if real, whether he can hold onto communicating with Them, a group of good monsters, and also protect himself from The Others, a group of destructive ones. These and others threads, such as IQ's search for a white rabbit, all flow into each other in the otherworldly 120 page climax.
Going in, I wasn't sure whether this would be more akin to Taiyo Matsumoto's long-form works (Tekkon Kinkreet, Ping Pong) or his more bizarre short stories and opaque narratives (Brothers of Japan, No. 5). Despite the wild third act, it's firmly grouped with the former, most directly invoking Tekkon Kinkreet: Yuki reesembles White; Makoto, Black; Ganz, the grandfather character; the troubling awareness of two different worlds that children experience as they grow older, torn between that of childhood of adulthood; the importance of human connections; etc. If these sound like crude simplifications, they are; it's also important that Matsumoto inverts these ideas, where here the anchor is Makoto, and it is Yuki who directly faces the surrealistic descent into himself and The Other Side. Even then my word choice isn't entirely accurate, either, for it's ascending through the school to a nonexistent floor that Yuki reaches The Other Side. This kind of duality is reflected everywhere in the narrative, whether in the repetitions (why Yuki likes the school office in the summer and winter) or in IQ' ambiguous relationship with Yuki and his almost metafictional actions and words in the narrative itself.
Beyond all that, though, is an emotional narrative for the first three seasons (and much of the beginning of winter) of the school year, with elliptical storytelling that becomes slightly grounded with the inclusions of dates. This already shows a stark difference in approach that Matsumoto takes from most manga-ka out there; after all, the story itself could be told in one-quarter or even one-tenth of the total pages (about 460, technically), but the sense of time and progression, as well as reflection (a lot of elementary school memories came back as I read this) is far too important. Compared to the final product here, a shortened version would just dull the impact.
It's pretty much redundant to say that Matsumoto's art is amazing, but he really outdoes himself here: it has the same refinement of Ping Pong, and the more ambitious scenes (IQ "dissolving" and his photorealistic eye; Makoto's first daydream and his entrance onto the many rooms of the fifth floor) better the best portions of Tekkon Kinkreet. The dozen or so darkened pages in the climax don't even bother me, as the eyes, faces and monstrous shapes are defined just enough to give detail, while also anticipating the feeling of liberation when we arrive at the wide open finish.
I'm not sure where I'd rank GoGo Monster against Matsumoto's masterpieces (Hanaotoko and the aforementioned 90s works), but that seems irrelevant; it's simply a damn good book that's required reading for anyone interested in genuinely unique storytelling. I know I'll be reading it again and again.