The live-action adaptation of Gintama exists to fulfill one purpose only: to bring to Japanese audiences, and fans overseas, a live-action incarnation of a beloved property. Everything outside of that is all but irrelevant. And the shock of it is how the movie does indeed reproduce, with startling fidelity, the off-the-wall, silly-then-sincere flavor of the original material. But don't even think about making this your first Gintama outing.
Underdogs, give it your all! ... tomorrow ... maybe
Let's start with the source material. Gintama, in its original manga incarnation, took a Meiji-era Japan setting, mashed up with some deliberately cheesy SF elements, interleaved that with clichés lifted freely from the yakuza/tough-guy/lone-wolf subgenre of '60s/'70s Japanese pulp cinema, and stood the whole thing on its head for laughs. Here, instead of the gunboat diplomacy that opened Japan to the west, the alien "Amanto" came along, confiscating everyone's swords and creating a new order that didn't have room for the romantic values of the samurai. (The potentially reactionary aspects of this story are a topic for another time, and for the most part the movie avoids them anyway.)
Gintoki Sakata was part of the failed anti-Amato rebellion, but since then has eked out a living as an odd-jobs man, along with his hapless sidekick Shinpachi and their bulletproof little-sister-esque comrade Kagura. Gin is lazy, messy, undisciplined, without any discernible marketable skills or ambitions. But get in the way of him or his friends — especially his friends — and he'll unleash a beat-down that makes it clear why they called him the "White Demon".
The first couple of scenes in the film recap the manga's own opening: Shinpachi, in a dead-end restaurant job, is rescued by Gintoki from being bullied by a couple of smarmy Amanto. Then, wham, we're thrown into a fourth-wall-breaking animated segment (deliberately badly animated, mind you) that tears up any notion of this being a mere live-action run-through of the first couple of manga installments. Gintama's anime adaptation had the same cheeky attitude towards its material: don't take this too seriously just yet, we'll tell you when you should do that.
And so we're pitched sidelong into some goings-on that, in classic Gintama style, at first seems to have nothing to do with anything. It's a caper where Gin and his buddies head into the forest to capture rare alien beetles that are allegedly worth piles of money, and run afoul of the Shinsengumi, the police force whom Gin constantly antagonizes without trying. (For those keeping score at home, this is a version of manga chapter 83.) Gin's life is made of misadventures in this vein — he tries to make a quick score, and ends up participating in a moral lesson of some kind, or instigating a general catastrophe, or (most commonly) both at once.
But of course there's an actual plot. It's a freeform adaptation of a major storyline in the original manga, where Gin is hired to find a sword that allegedly harbors terrible powers over its wielder. The details of the plot are actually not that important, if only because they were never all that important in the original Gintama anyway. What mattered was the overall flavor: Gin gets mixed up in a situation that is at first milked for satirical and/or non sequitur humor, which by degrees turns into some halfway to three-quarters serious statement about loyalty to one's friends, courage in the face of hopeless odds, or devotion to one's principles. In all cases, though, the humor's the alpha and omega: everything opens with a joke and closes with a joke.
The problem, as was the case with Gintama originally, is that the humor was and is a shotgun spray. I enjoy a dose of glorious nonsense as much as the next person (yes, you there, the one in the back in the ugly blazer), but that kind of humor always comes at the risk of falling entirely flat not just once but many times. To that end, most of the best gags in Gintama, this one included, are the random ones that were never intended to make sense. At one point a mute ducklike alien named "Elizabeth", one made up deliberately in the film to look like a guy in a suit, shows up in Gin's office ... and Gin comments not-so-surreptitiously about how this looks like some guy in a suit. (That gag actually has a great long-term payoff, where the whole question of what this thing is is toyed with hilariously.) But then there are interminable gags like the one where Gin visits a sword-forger and his daughter, which takes one not especially funny joke and stretches it so thin it becomes see-through.
Thing is, the original manga and anime functioned almost exactly like this, too. Not just in that the movie recreates the characters in live-action incarnations with uncanny accuracy (anime/manga-to-live-action veteran Shun Oguri is spot-on as Gin), or that it re-uses story elements fans will recognize instantly. It has the exact same unsprung rhythm in its humor, the same screwball-comedy-to-sincere-drama story progressions, the same ratio of bull's-eyes to duds with its setups and payoffs. It's rather startling how closely it tracks its source material — not the plot or the story, but the taste of it, the experience of encountering, being baffled by, laughing at, and finally nodding in recognition at the goings-on.
Audience, you know who you are
That all brings me back to the central problem: there's almost nobody to recommend this movie to except existing fans of the material, and they're pre-sold on it. If you know of anime but know nothing of Gintama, it's not guaranteed you'll get into it; some people like anime but despise non sequitur humor. If you know nothing of anime or Gintama, the barrier to entry is not merely twice as high but an order of magnitude so, and I hate recommending anything merely because it's weird for weird's sake. Okay, I take that back: a previous outing by director Yūichi Fukuda, another page-to-screen adaptation, deserves to be thought of in those terms: Hentai Kamen. It rises below weirdness.
Still, is there any real problem with saying this movie isn't for anyone but the fans? Maybe not. Much of popular culture now, and anime specifically, is about niches and niches of niches; you can spend your entire like as a Gundam fan without once trading words with a lover of (random title out of hat) Seraph Of The End. Likewise, it's disingenuous to insist that everything produced from it — including live-action adaptations — necessarily has to reach the widest possible viewership. There was no way something like Gintama would ever cross over to mainstream Western audiences, save maybe in the form of a one-time, you-hadda-be-there fluke along the lines of PSY's "Gangnam Style". This movie was made at the level it needs to be, for the audience that would care about it, and sometimes that's all that matters.
I've long been iffy about live-action adaptations of material that is too inherently cartoonish. That said, there's counterexamples galore that are forcing me to rethink my position. The live-action Batman TV series from the 1960s was cartoonish — and campy, and colorful — and unapologetically so. What was once corny kid's stuff is now pop art, in big part because of the way audiences have learned to see such things as part of a larger spectrum of possible approaches, instead of an absolute hierarchy where some things are better and some things are worse. It's not impossible to see how people would approach Gintama, and other resolutely dippy anime-to-live-action projects, with the same eyes.
But I think my original warning stands: Don't begin Gintama here. Start with a few episodes of the TV series or manga. Do a shot before you try to chug the whole bottle. Everyone else, you know who you are. You're welcome.