In Gintama the Movie: Be Forever Yorozuya (Yorozuya yo Eien Nare), why did Gin need to go to the future first?
He was infected by the White Plague during the Joui war, so why did the future Gin call him to the future to kill the plagued future Gin instead of just sending him to the past and preventing the plague from spreading in the first place?
Maybe because of the potential consequences of changing the past, Gintoki may not have met Shinpachi, Kagura and others at all.
Moreover, if you believe in the "parallel world" interpretation of time traveling, it can be said that the past cannot be changed, but only forked into alternative futures.
Thus maybe going to the future is the best choice here.
Gintoki is a lazy bastard, the only time we see him go out of his comfort zone, is when the people precious to him are threatened. And the level of effort he will put in also seems to be proportional to the amount of threat posed (like when the threat level is low, he will just save their ass while smiling smugly and stuff).
So, to convince Gintoki to put in all of his effort into preventing a future disaster like in the alt world, Gintoki needed to feel the sheer amount of suffering that would be caused if he failed; if he was just told some story, with a few videos; I doubt Gintoki would have risked his own life, as the story is just too damn far fetched.
A foreign virus that supposedly originated in the Joi war and stays dormant for 20 or so years, just to explode into the world, bombing it back to pre Amanto period? Yeah, is a very unconvincing story, unless you experience it, if you ask me.
