Bluesteel Blasphemer, Volume 1

Yukinari Amano loves guns. After reincarnating into another world, the young man goes on a journey with Dasa, the younger sister of the girl who gave him a second life. However, while on the road, the pair are attacked by a local deity—an "erdgod." No normal person could possibly defeat one—so when Yukinari uses his knowledge of guns and newfound abilities to do the impossible, he becomes worshipped as the area's new god himself!