OK, I think I’ve sorted it out. George and Easter/Esther Gregory from the 1850 census are almost definitely Daniel Gregory’s parents, and George Gregory’s parents (John Isaac Gregory and Sarah Leonard Gregory) came to Union County, IL, from near Charlotte, NC. There are some name usages (referring to Daniel’s brother George Henry Gregory as Henry Gregory, for example) that made this hard to follow, and entire families disappear in the 1860 census, probably because of concerns regarding the political situation. Given the typical thin-on-the-ground nature of official records from that period I think I’ve learned as much as I can without finding family records.