Gadiomnah was a city destroyed at the time of Christ’s death, named by the voice that was heard among the people during the three days of darkness. The voice listed Gadiomnah with the cities of Gadiandi, Jacob, and Gimgimno, saying that all of these had been sunk and turned into hills and valleys, and their inhabitants buried in the depths of the earth (3 Nephi 9:8).
The stated reason was the same given for the other cities in the list: to hide their wickedness and abominations, so that the blood of the prophets and the saints would no longer come up against them (3 Nephi 9:8). Gadiomnah is not mentioned anywhere else in the record.