If online research has been your sole/primary focus todate, you might never solve that brickwall! Strong possibility his parents were G. T. Pierce/Sarah LNU? In 1880 Somervell-Precinct 1-146-Image 14, G. T./Sarah can be found. The next forward image has Moses/Sarah. That's the only enumeration of G. T. Pierce. In view of his age in 1880, he may have died in Somervell? Proving kinship will probably require research of probate records? FYI a couple of family trees agree with my assessment.