A gate appears in Tokyo's Ginza district sometime in the 21st century. From the gate pours out monsters, knights from middle-age Europe, and other fantasy-like beings, and they kill many of the citizens of Tokyo. This event is known as the Ginza Incident.
The government sends a small group of soldiers from the Japanese Self-Defense Forces to the alternate world beyond the gate. Led by otaku soldier Youji, they find that the villages in the world are being attacked by a dragon. An elf girl who is a survivor from the dragon's rampage joins the group in their travels across the dangerous new world.
Links:
Artist's Twitter
Artist's Pixiv
Alternative:
GATE; Gate - Au delà de la porte; Gate - Jieitai Kare no chi nite; Gate - Where the JSDF Fought; Gate Jietai Kare no Chi nite, Kaku Tatakeri; Gate: Thus the JSDF Fought There!; Врата: там бьются наши воины; เกท:หน่วยรบตะลุยโลกต่างมิติ; ゲート 自衛隊彼の地にて、斯く戦えり