On a certain summer day in the year 20XX, in broad daylight a gate to another world appears in Ginza, Tokyo. An army and a swarm of monsters come storming through the gate. The Japan Ground Self-Defense Force is able to defeat the invaders and they head through the gate into uncharted lands. An epic fantasy about a battle between the human race and another world filled with dangerous enemies and beasts! *Source: Alpha Manga*

In the year 20XX, a mysterious gate to another world suddenly opens in Tokyo’s Ginza district. From beyond it pour invading armies and monstrous creatures from a fantasy realm. Japan’s Self-Defense Forces repel the attack-and push forward through the gate into the unknown territory on the other side. Among the soldiers sent to investigate is Yoji Itami, a 33-year-old otaku officer who is anything but a typical soldier. Leading his unit into the newly discovered world, he sets out to scout nearby villages and assess the situation beyond the gate. What awaits them is not only danger¿ but the sight of fantastical races and beautiful girls straight out of a dream. Thus begins the unprecedented encounter between modern military force and a medieval fantasy world, where guns meet dragons and diplomacy clashes with magic. *Source: Hanashi Media, Vol. 1*

A spin-off focusing on Princess Pinya's (from "Gate - Thus the JSDF Fought There!") life before the "Gate" opening to the other world event.

After graduating from their constant fighting, some magical girls were unable to keep up in class, got poor grades, and became NEETs. In order to help these poor former magical girls, the cleaning company "Mei Company" was founded! Will swapping their magical staffs for cleaning tools help to rehabilitate them? Gate protagonist Itami's beloved doujin manga about former magical girls cleaning up messes.

In August of 20xx, a portal to a parallel world, known as the "Gate," suddenly appeared in Ginza, Tokyo. Monsters and troops poured out of the portal, turning the shopping district into a bloody inferno. The Japan Ground-Self Defence Force immediately took action and pushed the fantasy creatures back to the "Gate." To facilitate negotiations and prepare for future fights, the JGSDF dispatched the Third Reconnaissance Team to the "Special Region" at the other side of the Gate. Youji Itami, a JSDF officer as well as a 33-year-old otaku, was appointed as the leader of the Team. Amid attacks from enemy troops the team visited a variety of places and learnt a lot about the local culture and geography. Thanks to their efforts in humanitarian relief, although with some difficulties they were gradually able to reach out to the locals. They even had a cute elf, a sorceress and a demigoddess in their circle of new friends. On the other hand, the major powers outside the Gate such as the United States,China and Russia were extremely interested in the abundant resources available in the Special Region. They began to exert diplomatic pressure over Japan. A suddenly appearing portal to an unknown world—to the major powers it may be no more than a mere asset for toppling the international order. But to our protagonists it is an invaluable opportunity to broaden knowledge, friendship, and ultimately their perspective towards the world. (Source: Baka-Tsuki)

A 4-koma set in the "school" at the Alnus base.

Satoru Sao manga adaptation of the second Part of the GATE's Light Novels.

Captain Gorou Edajima, a seasoned captain who has visited the various countries of the Special Region on a research mission, and master cook Petty Officer, 1st Class Hajime Tokushima were given a special mission: rescue an American journalist kidnapped in the Special Region! Aboard an Oyashio-class attack submarine, they set out onto the seas of the Special Region, but little did they know their mission would lead them headfirst into a whirlpool of petty rulers and pirates vying for control in the Blue Sea... (Source: AlphaPolis, translated)

A spin-off of Gate: Jieitai Kanochi nite, Kaku Tatakaeri series featuring three main heroines as up-and-coming idols and Itami as their manager in a world without war.

The fox-girl Foxy was feeling down because her troop was still in debt despite fighting as mercenaries. Now Foxy and her friends are traveling in a tank, hoping to earn money through trading, and searching for a legendary plate of great value. Along the way, they happen to rescue a kidnapped girl, Leona, and learn that she has information about the plate and is a skilled trader, too. Foxy is invited to join Leona's caravan as a reward for her rescue. Suddenly, however, the caravan is attacked by mercenaries and destroyed. Burning with vengeance, Leona decides defeat her enemies through economics, instead of taking on their greater military strength. As a tense trading battle unfolds on the commodity exchange, Foxy and his friends, who are on separate missions, have obtained important information that could move the market... (Source: MF Bunko, translated)

In the summer of 20XX, a "gate to another world'' suddenly appeared in Ginza, Tokyo. What came pouring in from inside was a mysterious army and a group of hideous monsters. They go around slaughtering people, and Ginza in broad daylight quickly turns into a hellscape. While everyone was helplessly thrown into confusion in this state of emergency, there was one man who was brave enough to move around. That man was Yohji Itami, an otaku Self-Defense Force officer who would later be called the "Hero of Nijubashi"!