Drawn by Sorachi Hideaki (空知英秋) sensei, Gin Tama is set in a rather unique background where at the end of the Edo period, circa 1850s, the country of Japan was forcibly "liberated" by aliens with their ray guns and space ships. In actual history, feudal Japan was forcibly "liberated" by the Americans with their, then, technological advanced steam ships and cannons.
Comedy ensues when alien cultures clashed with the earthlings'. When a local populace still barely out of the iron-age gets hit with modern comforts like internet, air conditioning, hover cars, smart phones and daily soap dramas.
This is the setting where the author attempts to tell stories revolving a small band of misfits consisting of a kind hearted boy who wants to reinstate his father's kendo dojo. And an alien girl who looks Chinese, has immense strength, incredible kunfu and a bottomless pit for a stomach. The two are led by a jaded, strawberry milk addict, ronin named Sakata Gintoki. The 3 form the Yorozuya (万事屋), house above a snack bar and doing odd jobs in Edo but mostly in the township of Kabuki-cho.
Gin Tama began its serialization in 2003 as an attempt to capitalize on the, then, revival of samurai culture and periodic drama on TV. The author, Sorachi sensei, wasn't particularly interested in doing something serious nor periodic but had to listen to his editor. In the end, he came up with a samurai-scifi-gag idea and thus Gin Tama was born.
Initially, Gin Tama was criticized for its art work but soon people started appreciating his story telling. You see... the author Sorachi sensei has the ability to work the conflicts and nuances of modern life into his comics very well and yet keep up the gags, humor and drama. It is often said (at least of the anime) that Gin Tama has the ability to make you cringe, curse, cry and then laugh in 3 minutes and I can attest to that on more than 1 occasion.
Anyway, if you have time to spare and need a good laugh, I highly recommend Gin Tama. The anime recently went into an indefinite hiatus but there is still 265 past episodes to catch up with. Have fun!