Henry does not appear to have been a common Gainer name in WV.There was a Henry H., age 42 in Dekalb Dist., Gilmer Co. in 1880 Census.Wife: Surrena, age 36; children: William, 15; Lele Ellen, 9; Hattie R., 5; domestic: Mary A. Kelly, age 18.I've not found evidence of his relations.
I rechecked Barbour, 1880 for Henry.Find none, but William A. Marsh has them all spelled "Ganer."
There is a Brien Gainer, age 89 with wife Mary, age 70, b. OH in 1880.Could this be your Brian & Polly?I cannot explain a Brian of this birth year (1791) -- too young for the Bryan b. 1778 (wife: Nancy or Rachel or something) and I don't believe that George & Sarah's Bryan could be near this old.
I never hear questions about them, but there were Austin Gainer (wife Missouri) and James B. Gainer(wife Mary E.) in Doddridge in 1880 (I believe brothers).
Also, the Levi Garner (wife Ellis Ann, perhaps Lettis Ann) in Gilmer may be a Levi Gainer that left Barbour after 1870. (Note 12 year old Stalnaker as that surname was already used by the Gainers as a given name.)
Benjamin Genoy (?) (wife Lucinda) of 1880 Calhoun maybe Benjamin Vanoy.
Do you have any ages on your Bryan & Polly? Or other clues to be watching for?