I found your family tree and I think you've found more documentation since you posted this request.
I assume that Mary/Polly was mentioned in Thomas Gillespie's will since you listed it as a source.
South Carolina didn't start recording births until 1913 so that option is out. If you know where she was born, you might find her church/baptism record.
Polly A. Gillespie
John H. Jones
Source: Marriage and Death Notices, Southern Christian Advocate & South Carolina Compiled Marriage Index 1641-1965
If you can find their actual marriage license, both sets of parents might be on it. If you can find the date and location, look for the marriage announcement in the newspaper. Both sets of parents might be included.
Prior to 1850 only the head of household appeared on Census records.
Do a Google search for the DeKalb county Georgia genealogical and historical societies and ask them for a copy of her death notice. Her parents names might be there too.
One other piece of info that I found about their son.
Jordan Marion Jones
Born: 4 Feb 1842, Dekalb Co. GA
F: John H. Jones, b. SC
M: Polly Gillespie, b. SC
Informant: R. E. L. Jones, Conley, GA
Buried: Macedonia Cemetery, June 29, 1925
Source: GA Deaths 1914-1927
Look for John's death notice and probate records for a list of their children. Successful Searching.
Died: 28 June 1925, Conley, DeKalb, GA