So, this "trailer" is basically a few minutes of Gintoki, Katsura, Sakamoto, and Takasugi in the Joui war, and the rest of the time is taken up by the characters complaining that Gintama hadn't gotten a movie yet (by 2008) and other characters complaining that they want more screen time.
This short was awesome in the beginning and funny at the end. It does a good job of showcasing why Gintama's comedy works so well. The appearance of all the major side characters on screen served to show that they make the series what it is. The fake trailer for a movie about Shiroyasha, though, was next level cool. If the viewer watched this after the first 201 episodes of Gintama, this trailer makes it seem as though there is a lot left to unveil going forward regarding Gintoki's past. It did its job of seeming like a badass movie trailer, and it's really fun to watch, so I recommend it.