Gintama stops pretending to be a show and just makes a fake movie trailer for ten minutes, which means the joke is that there is no joke, just chaos with a budget. It's the show at its most honest: a bunch of comedians with animation deciding that the best use of your time is their time wasting it on purpose. If you've made it this far into Gintama, you already know this is the only thing that could possibly happen here.
Shiroyasha Koutan is a ten-minute Gintama short made to look like the trailer for a movie that was never actually produced. It first screened at Jump Super Anime Tour 2008 and went without a DVD release until Aniplex set one for September 30. That disc also includes the Gintama OVA shown at Jump Anime Tour '05. Expect the franchise's mix of action, comedy and science fiction in a compact package.