I don't know much about the Ganns, but they are a collateral line of a collateral line --
I list a Marion Basel Gann b prob after 1878; parents Nathan Marion Gann and Margaret Alameda Cox b April 07, 1862 Comanche Co, TX d December 22, 1922 Burkburnet, TX; married in May 1878 in Chico, TX.Nathan and Margaret had 10 children.
Margaret was the daughter of Basilee Pierce Cox of Collin Co TX (son of James Silas Cox of KY) and Delilah Fisher of Fannin (later Collin) Co TX.Delilah's nephew, John "King" Fisher, was a famous outlaw and later deputy sheriff of the Uvalde/Nueces Strip areas.The Fishers were intermarried with the Damrons, Ratliffs, (and by extension, the Brocks), Thompsons?, Eastwoods, DeGraffenreids, and others from Newton, Johnson, Barry, and McDonald Co MO.They all came to TX early (many about 1844 to Fannin/Collin and Ellis Co) and seemed to move together in family clumps.Many of these folks (Damron, notably) moved toward Jack Co before the Civil War.
After the Civil War (and some of them by 1850), most of the families moved toward Goliad and to Caldwell Co, with some towards Travis and Williamson Co.Then many moved west to Kerr, Uvalde, etc.
My point being that the Ganns may well have been associated with this group.(I also have wondered whether Gann might be a shortened version of Gannaway, as the Damrons and some associated families intermarried a lot with the Gannaways back in Green/Adair Co area of KY before going to MO.)I would sure look in Collin, Caldwell, Goliad, and Williamson Co for the Ganns, and possibly in Newton Co MO as well.