DemographicsGretna is approximately 126.44 square kilometres. In 2016 the population of Gretna was 211. The population is 50% male and 50% female. On average the monthly mortgage repayments in Gretna are $1000 to $1399. The most common weekly income bracket is $300 to $399 per week. Weekly rents in the area are on average $225 to $275.