Your information certainly leaves out the possibility that Gabriel Alexander in Rowan/Iredell County, NC is the son of Gabriel (senior) of Augusta County, VA! So I will let that rest.
I have poured over the wills for John Alexander d. 1764 Chester County and his daughter, Rebecca Emmit, d. 1792 Chester County trying to figure relationships. It is clear that Rebecca had brothers named William, John and Gabriel. She also had sisters: Mary Alexander m. Joseph Mehaffey and lived in Cecil County; Martha Alexander m. William Porterfield and lived in Berkeley County, VA. There is no way to know if she named every one of her siblings. And her father, John, had named ONLY Rebecca and his wife, Susannah, in his will. Not at all helpful! We can only guess at why Rebecca was so favored at the exclusion of every one of her siblings!
There was also a Gabriel Alexander mentioned in a survey dispute in Chester County in 1726. This is not likely to be the same one that was in Augusta County since the Augusta County Gabriel would only have been 16 yo when the dispute began. Further more, in 1727 Abraham Emmit granted a warrant to arrest a couple of guys for trespassing on the complaint of Gabriel's son, Zaccheus. That means that the Chester County Gabriel had to be old enough in 1727 to have an adult son. Hmmmm. Doesn't seem as though he could be the same one who was Rebecca (Alexander) Emmit's brother. Maybe her uncle.
I think I'm going to have to look more closely at Chester and Cecil County deeds.