Garrettsburg, MissouriGarrettsburg is an extinct hamlet in Buchanan County, in the U.S. state of Missouri.HistoryA post office called Garrettsburg was established in 1872, and remained in operation until 1902. The community has the name of Zach Garrett, a pioneer citizen.GeographyIt was located in northwest Tremont Township along the Platte River midway between Agency and Easton.