Gretna is a city in Sarpy County, Nebraska, United States. It is part of the Omaha metropolitan area and considered a suburban bedroom com…Gretna is a city in Sarpy County, Nebraska, United States. It is part of the Omaha metropolitan area and considered a suburban bedroom community for Omaha. The city's population was 5,083 at the 2020 census and estimated to be 9,071 in 2022.