Worm (Parahumans #1) - Chapter 11: Insinuation 2.4

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

This story is centered around the man named Yi Ji-Hyuk, who has spent 1000 years in a Medieval Western fantasy-like world before returning back to the modern-day Earth. When he returns, he finds that his home is facing monster threats, and that humans with superpowers have appeared to protect the innocent people of Earth.

Always looked down upon by others, Ji Feng gains technology far more advanced than the present, as he discovers its abilities, he begins to walk a path far from ordinary. With X-ray vision allowing his to find jade stones in rocks. With this futuristic technology, Ji Feng accomplishes something no-one can compare to. This is the beginning of a legend.

- Description from Novelupdates