I too am a Gilliam researcher and I doubt that John Gilliam who married Sarah Faris is the John Gilliam born in 1733 in Surry County, VA.
Two of John and Sarah's daughters married in 1767. (Cecelia married Edward Cox 10 Dec 1767 and Mary married Charles Sneed 22 Dec 1767). It leaves only 34 years for the two generations. In other words, John would have had to have married Sarah at about age 17 and had his two daughters immediately after his marriage who would then have had to have married by age 15. (18 months for the two pregnancies.) Also it is believed that Cecelia and Mary were not among John and Sarah's first born. It is generally believed that sons James and John were among the first born.
The John Gilliam who was born in 1733 in Surry is the son of Hinchea Gilliam, Jr and Faith Briggs, his wife. This John married Amy Cook the daughter of Reuben Cook. This John left a Will in Brunswick County, VA dated 27 Jan 1806.