I visited Gretna last summer. On a sign as you enter the town.......
CITY OF GRETNA
Incorporated 20 August 1913.
John Ehret, First Mayor.
Seat of Jefferson Parish Government since 1884.
German settlement laid out in 1836 by Benjamin Buisson for Nicolas Noel Destrehan as VILLAGE OF MECHANIKHAM.
John Ehret was a brother to my Gt. Grandmother Catherine EHRET.
I was always perplexed looking at the census too. I hope this helps.