GATE: Jieitai - Kanochi nite, Kaku Tatakaeri (Novel)

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

Note: The volumes are paired and the chapter numbering resets every 2 volumes.