Sorry I missed your post last night.
I am not familiar with them in particular.
My knowledge tapers off after 1783 and also when it comes to non-military rifles (which is most of flintlock era rifles).
In addition to the book you mention, there are three Gillespie rifles shown on the American Long Rifle forum. Two have detailed explanaitons of their characteristics.
[URL]https://americanlongrifles.org/forum/index.php?board=112.0[/URL]
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In general I don't think you'll find original long rifles before the 1790s with the deep curvature in the butt.
Compare with 'Jacob Bower's rifle made in the 1760s, probably by George Schroyer in Reading, Pa.
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[URL]https://emuseum.history.org/ex...st-an-enemy-weapons-of-the-revolution/objects[/URL]
WV64Signet's question about the family migration story is a good one. There was a strong connection between the Southern Appalachian region through the Shenendoah to the Lancaster Pennsylvania area and on to Philadelphia. A lot of commerce (cattle for example) and migration along the Great Wagon Road as it was sometimes called. So yes they could have moved inland the tidewater, or had connections from much further north.
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Things to look into for the Gillespie rifles might be their sources of parts and materials. It took a pretty big investment to get into making barrels and locks. To get all the parts made under one roof required tremendous knowledge, equipment and raw material. As the description indicates a water powered mill, that suggests to me they were running a trip hammer or some other equipment that needed serious power.
Besides Colonial Williamsburg, I'm not sure of where else you could go to see barrels forged and rifled the way they were in the 18th century. If you go, check ahead to find out what they are working on. They don't have a begining to end in one day demonstration, but I do beleive they have a schedule or some way to follow the processes from raw materials to finished rifle.