Thanks for your input, Don.
I see the discrepancy and after researching further, realize these are two Mary Polly Griffiths. I should have known with a name like Mary Polly that there would be more than one in the area. The Mary Polly that married Granville was probably daughter of Alexander Griffith and Rachel Hodge. My Polly (1799-VA) was possibly sister of Alexander, daughter of George Griffith and Nancy Byrd.

It is possible that my Thomas Gibson is brother of Granville. Here are notes lending themselves to that theory:
1) William and Thomas seem to be only Gibsons in Hopkins county near the time of his marriage (1817).

Wm Gibson, 160 acres on Drake Creek, Hopkins Co, Ky, 8/27/1819 - from Ky Land Grants, Book 26, Vol I, Part I, Chap IV Grants south of Green River.

Thomas and Polly married 10 Nov 1817, Hopkins Co, Ky

Home 1820 for Thomas and Polly:
Hopkins Co, Ky
Thomas 1794-1804
Polly 1794-1804
2 sons 1810-1820
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Home 1820 for Wm Gipson
Caldwell Co, Ky [neighboring Hopkins Co and Christian]
William - before 1775
Wife - 1776-1794
1 male 1804-1810 - Granville?
1 male 1810-1820 - John Jackson?
2 females 1794-1804 -
1 female 1804-1810 -
1 female 1810-1820
Wm and Nancy's daughters are reportedly, Eliz-1807, Telitha-1811, Matilda-1812, Penelope-1816, and possibly an older Nancy. Weak point of this theory: Girls' birth years don't fit the census.
(Thomas is married and out of home)
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2) Contemporaries to members of associated families:

On the 1820 Caldwell Co, Ky US census noted above, next door practically are family members of both Mary Polly Griffiths: George Griffith, Bird Griffith, Gipson Gray [married Rachel Hodge Griffith after Alexander Griffith died], and other Grays.
Further up the page, John Entrekin family. This latter was reportedly William and Nancy's co-in-laws - father-in-law to daughter Elizabeth Gibson,b 1807 (married Asa Entrekin).
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3) Thomas and (mother?) Nancy, only adult Gibson/Gipson heads of family in Calloway, KY:
William not found [died 1825?]
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Home in 1830:
Calloway Co, Ky
Thomas Gipson - 1790-1800
Polly - 1790-1800
John - 1815-1820 [born 1818]
2 males 1820-1825
1 female 1820-1825 (Jane Holland Gibson, 1823?)
2 females 1825-1830 (Susan-1829 and Rachael-1830)
Cebert (Siebert) Griffith family same page - possibly brother to Polly.

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Next page on 1830 US Census - Calloway Co, Ky
Nancy Gibson, 1770-1780
1 female 1815-1820 (Penelope?)
1 male - 1800-1810 (Granville?)
1 male 1810-1815 (John Jackson?)

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(On same page)
Rachel Gray [Mary Polly's mother - widowed after 2nd marr to Gipson Gray]
2 females 1810-1815 [probably the Mary Polly who married Granville]
etc...

So, is that about as clear as mud, or what? Maybe the 1820 Caldwell Co William is the William who died in Marshall Co, Ky 1825. If so, he lived for awhile in Tenn, according to places of birth for the children - up to 1816. Or could be he came directly to Marshall Co after 1820. Then, there's a William Gibson in Butler Co, in 1810 which also could fit William of Caldwell Co-1820.
And then we have Cassie/Casey Gibson you mentioned and her possible connection to Cassandra Gibson, sister to the William of Onslow Co, NC.
If you see anything that connects to my Thomas Gibson, ca 1798-NC, please get in touch. Again, he is found most clearly in 1850 US Census, Graves Co, Ky. I have not found Thomas or Polly in the area after that date. Male children are: John (1818), Martin (1835), Thomas, Jr (1838) Female children: Jane (1823), Susan (1829), Rachael (1830), Elizabeth (1842). Related families: Whitis, Griffith. He was active member/Elder of Mt Pleasant Primitive Baptist Church in Graves Co as of 1834. 1820-Hopkins, 1830-Calloway, 1834, 1840 and 1850- Graves, Ky.

I had previously looked at Elijah Gibson, of Graves Co, Ky, as possible father to Thomas. I am now seriously questioning that. Although he also can be traced back to Onslow Co, NC. Do you know anything about Elijah?

Any information or advice is welcome.
~Kathryn