Based on a super popular series, boasting popular actors and actresses, visual that is faithful to the original anime/manga as well as period drama, great box office performance in Japan: all the recipe needed for a good anime/manga adaptation. Although I am never follow the original series, I decided to watch it even without reading reviews, whether for the plot or the movie.
I feel that the movie will be have contrasting reviews depending on whether the person watching is a fan to begin with. I don't follow the original story so I find the plot is quite weak, especially considering Gintama is more like a gag series. I personally enjoy over-the-top performance to replicate the over-the-top expressions and scenes from anime/manga. I also find the references to another series (that is what makes Gintama quite unique for a comedy) hilarious because I am familiar with the series they are referring to. However, for non-fans, they will find the comedy lame and they will not get the references or understand why the fans are laughing.
The setting is kinda lame right from the start as Japan is invaded and ruled by aliens. Even so, I think they really cannot be bothered about the plot to begin with. The rather weak plot gets further thinned down with all the comedy elements in between that people looking for a good story will find themselves in a "WTF is this movie" kind of feeling. But then again I don't know if the plot follows the original anime/manga that fans will like it.
I can close an eye to the story since this is meant to be a live-action of anime/manga which usually is average at best in general. The thing that I can't stand the most is the CGI. The CGI is painful to watch and it is as if the movie producer does not even try to blend the CGI to reality. The CGI looks obviously like a computer work added in a post-production stage. Ouch.. I think not acceptable given the technology today. Even Power Rangers in the 90s is not this awful.
Gintama plays its trump card well with the comedy and self-depreciating jokes throughout the movie. For example, Gintama said that Elizabeth cannot be an alien and must be a human wearing a cosplay wear and a few moments later we are shown with Elizabeth falling down and the human legs showing. The casts sing the initial opening and closing song in a "can't be bothered style". In fact the initial opening scene is like a karaoke session with the karaoke video only showing Shun Oguri's name (the real actor name who plays Gintoki) all over as if he is the only cast in the film. But ultimately, just like the discussion between cartoon Gintoki, Shinpachi, and Kagura at the start of the film, it will be difficult to win over new fans with this live action movie.