Here's what I did today. I looked at the Gibson's who were in York in 1779 and saw that all were still in York in 1780 and 81. I sent you the wrong info on Fawn. In 1779 it was James and Robert not James and John.
then I figured that your Gibson was gone from Fawn by 1779 so I looked at the taxes feom 1771-1778 and there were only 2 Gibsons in Fawn for those years Robert and Jacob. Robert died 1778 and I believe the James and Robert in Fawn in 1779 are his sons. Jacob sold land in 1779 to Mathew Hartford per Deed 2A227. Jacob's wife was Anna Whiteford per her Mother's Harford County Maryland will in 1791 in Book AJU page 240. Anna was the daughter of Hugh Whiteford and the widow Anna Alexander. Her first Husband was Jedediah Alexander who died in 1750 and had no children and he had York County will A-10.
This still doesn't prove who James parents are. I feel some of the older sons of Robert or Jacob were living at home with their wives and your James might be from one of them.
Jacob's 1809 York County M-151 will does not list a William or Sarah or James.
How positive are you that York County is the right place to look?