According to Amelia County Virginia Chauncery court records, I believe that the Mary Gill most researchers have listed as married to John Old was actually born Mary Wilson to John Wilson Sr and Mary (?possibly Mary Cousins?). One court record William Wilson vs John Wilson documents her as the daughter of John Wilson and lists her once in the document as Mary Gill and on another page as Mary Old. It appears that Mary first married John Old who died in 1768 and by 1772 she was married to Daniel Gill. That is why some court documents will list her as Mary Gill and others still as Mary Old.
On page 3 of Edward Old vs James Old it lists Mary Gill as John Old’s widow. “ showeth unto your worships that part of the estate of their late father John Old that was allotted and laid of for the dower of their late mother Mary Gill, widow of the said John Old ….”
On page2 of William Covington vs Exectors of William Old .September 1772.”we command that you summon Daniel Gill and his wife Mary Executor and James Old and Edward Old Executors of John Old descd.”
Page 6 of the Covington vs Old, September 1772 “ Daniel Gill and Mary his wife, extrs of James Old and Edward Old, exts of John Old descd.”
Page 11: “..and that Mary Old executor of said John hath intermarried with a certain Daniel Gill of this county…”
The Chauncery Court records noted above, along with a photocopy of John Old's will are attached to my family tree on ancestry.com. I hope this info is helpful in your search.