As translated by Twitter user Kewl0210, Gintama creator Hideaki Sorachi wrote a one-page letter to fans explaining the situation.
While Gintama won’t be in Weekly Jump, Sorachi wrote, the manga isn’t ending. The reason, he continued, is that Jump has a system in which manga creator tells the editorial department half a year in advance that the story is coming to a close. At that point, it all starts wrapping up.
“But with Gintama, I misread this and for the past three years now I’ve been saying, ‘I don’t think I can wrap up quite yet,’” Sorachi wrote (via Kewl0210).
Thus, Gintama will be published in spin-off mag Jump Giga, where the manga can apparently get more pages, while Sorachi wraps up the story.
“I honestly would’ve preferred to bring you the story all the way to the end in Jump,” Sorachi wrote (via Kewl0210), “so I really have to apologise for screwing the whole thing up.”