This looks like your couple on the 1880 census Warren Co, TN, neighbor to Cannon Co.Looks likeher name may have been Sarah Malissa(?)I see you have other lines in Warren Co:
Category: Name;Relationship to head of household; Marital Status; Gender; Race; Age; Birthplace; Occupation; Father's Birthplace; Mother's Birthplace.
Francis GUNTERSelfMMaleW27TNFarmerTNTN Sarah M. GUNTERWifeMFemaleW26TNKeeping HouseTNTN Barbary E. GUNTERDauSFemaleW5TNTNTN
1900 census Hill Co, TX:
Gunter, Francis, h, b Jun 1852, 47, M 26 yrs, b TN, TN, TN " Sarah M., w, b Nov 1853, 46, M 26 yrs, 7 ch, 6 lvg, b TN, TN, TN " Charles, s, b ? 1885, 14, b TN " Sallie, d, b ? 1889, 10, b TN
Several other Gunters in Hill Co, TX on this census, probably older sons out on their own.You might track them down, and see if anyone is working on any of them.You never know when another descendant may have the one clue you need.
Have you checked the Warren and Cannon Co, TN GenWeb sites?See what records they may have online and be sure to post your query on there, too.