So I saw the genres of Gintama and I found this:
It's a shogun-era (Edo-period) animé and they use katanas instead of huge weapons; there are also some mobile-phones, etc. But what’s so 'Sci-Fi' about it, except for the fact that there is inter-space travelling, which is obvious given the plot of aliens invading? I don't know why it is categorised as a 'Sci-Fi' animé.
As mentioned on the Wikipedia article on Science Fiction:
Science fiction is a genre of speculative fiction dealing with imaginative concepts such as futuristic settings, futuristic science and technology, space travel, time travel, faster than light travel, parallel universes and extraterrestrial life.
Hence, Gintama is a Sci-Fi anime based on its:
Futuristic Settings & Extraterrestrial Life:
Gintama depicts Edo after it had been conquered by the aliens, and how it looks after they brought modern-day technology into a samurai country of ages past.Source
Inusei Amanto have been around in the Gintama world for over 20 years and have brought with them modern technology and led to new laws being enforced such as the ban on swords. Many other races of Amanto (aliens) have been introduced throughout the series such as Yato tribe, Shinra tribe, Royal tribe, among others.
There are even floating mobile cities in the series such as Dragon Palace City (before it descended into the sea and remained there forever).
Futuristic Science & Technology:
The items featured in Gintama range far and wide from traditional japanese items to incredible sci-fi machinations.Source
Most futuristic of the items shown in Gintama series has to be the presence of fully functioning maid-robots like Tama. While robots do exist in our time, they are not yet available on sale for household-purposes, and none are as functional as Tama.There was even a robot named Saburou which was built by Hiraga Gensai.
Another aspect of futuristic science and technology is seen in making of space shuttles, which are a very common occurrence in the Gintama series and enable space travel as mentioned below.
Space Travel:
While only shown a few times in the series, space travelling is common throughout the series with Amanto, along with some humans in space travelling all the time from one location to another. It's so common that there even exist "Space Pirates" Harusame
Hence, while Gintama does not revolve around the Sci-Fi genre like other Sci-Fi anime such as Steins;Gate, the elements of Sci-Fi are present throughout the series. As put here:
Being based in ancient Japan after its invasion by aliens, the series manages to mix more historic items such as katana and traditonal style architecture with modern ideas like convenience stores, mopeds and television, not to mention the rather liberal application of science fiction, primarily Edo's terminal station which features high tech design, numberous airships nearby and some form of teleporter system. Over all, Gintama somehow manages to bring together these three styles into the working world of Gintoki and his friends.
Whenever dealing with futuristic stuffs, technologies, extraterrestrial lifes, it is categorized as "sci-fi". If you ever watched Gintama before, you should aware that almost every episode has a short prologue. You can see tons of futuristic stuffs going on, like spaceships and aliens.
Still not convincing enough?
How about Space Scene?
Some Weird Looking Aliens and Robots?
Or Futuristic Technologies?
Keep in mind it's just a minor & background settings for the whole series, the main genre is still Action and Comedy.
