Gretna is a city in Gadsden County, Florida, United States. The population was 1,460 as of the 2010 census, down from 1,709 at the 2000 cens…Gretna is a city in Gadsden County, Florida, United States. The population was 1,460 as of the 2010 census, down from 1,709 at the 2000 census. It is on U.S. 90 approximately 8 miles south of the Florida-Georgia border.