I can't substantiate this but I am told my another Grable researcher that all of the Grables in the U.S. descend from one Peter Craple who originally settled in Frederick, MD in the late 1600's.
From there the family moved to Washington Co., PA where there is a large reunion every summer. That branch of the family has done a lot of research and even published a book on the Grable family.
My grandfather, James Christian Grable, 1897-1989, WV was a descendent of that line. If you have access to aol message boards they usually have connections to the Grable researchers. Good luck!