The anime had ended in March, but Shinji Takamatsu, one of the main directors of the anime, had said at an event later that month that "Gintama is absolutely not over. It hasn't even begun yet! It will definitely return."
The story of Gintama takes place in Edo (known today as Tokyo because of its name change in 1868), Japan, which was conquered by aliens called Amanto in the late Edo period. The samurai of Japan fought the aliens, but after their defeat the Amanto placed a ban on carrying swords in public. The plot is focused on an eccentric samurai, Gintoki Sakata who helps a teenager named Shinpachi Shimura save his sister Tae from a group of aliens who want to make her part of a brothel.
(Source: Anime News Network)