I Was Banished from Heaven, Now I'm the Demon King's Stepdad?! - Chapter 28 – “Rin vs. Rin: Before the War Starts”

Long Fei, a cross-timer and doujin, is an otaku who traveled to another world. In the original world, he was an otaku fan of Gundam and military science fiction.

He died in an accident and was reincarnated into another world. The Gundam works he had read before his death and the techniques in various military novels could be brought to another world.

How would he use the Gundam that Long Fei created? What waves can be caused in this different world?

- Description from novelbuddy

Stranded in a world not his own, in a body not his own, Kaden finds himself stuck as a low rank marine in treacherous waters in the middle of an ocean where only the strongest can survive and a count down until a war that would shake the oceans and the very world itself erupts, where the kings of the sea will devour the chum. Gamer System. Insert.

He rose from hardship, but was felled by betrayal. From then on, one man, one gun; treads the path between Evernight and Daybreak to became a legend. No matter what was destined to be his fate, he intends to become the ruler who dictates.

In the foreign world, the martial artists dominate the world and the sword of the emperor controls the fate of hundreds of millions of lives! Long Yu, the number one killer on Earth, after dying, takes rebirth in this new world in an aristocratic family of martial artists but faces rejection because he is unable to practice martial arts.

- Description from Novelupdates