You mentioned that Gabriel b. ca 1710 and d. ca 1778 in Augusta County, Virginia had a son named Gabriel. Do you know anything about him? When he was born and where he lived?
I am wondering if this is the same Gabriel who later was in Rowan/Iredell County, North Carolina. He was b. in 1743 and d. 1806. I don't remember just when he appeared in Rowan County but do remember that he must have been somewhat young. In his twenties as I remember.
You do not have a mention of a Josiah Alexander as a son of Gabriel (of Augusta County). Is there any reason to think that not all the sons are known or any were left out of the will? Rebecca (Alexander) Emmit of Chester County, Pennsylvania mentioned nephew Josiah Alexander, son of Gabriel.