I doubt this John Girty is related to Simon Girty. The only known sons of Simon Girty Sr. and Mary (Eckerlin?) Newton was Thomas, George, James, and Simon. Simon Sr. was murdered in 1750 by Samuel Saunders, and then his widow married John Turner. Mrs. Turner then had John Turner Jr. about 1753 or 1754. John Turner Jr. often went by John Girty, but he never had any children of his own and died in Peebles Township, Allegheny County, PA in 1840.
The Girtys are believed to have had a sister who married a Brouster and who had a son named Simon Brouster. Simon B. went to Canada to live with his uncle Simon Girty and after grown headed west, so the family story goes.
There were other Girty families living in PA and MD early in the 1700s and 1800s, not sure how all are connected if they are. I hope this helps a little.