Probably not the last time that I post a link to a Jas Townsend cooking show. I am going to suggest that biscuits & gravy is a Victorian invention. Chemical leavening (Part 1) began with potash in the very late 18th century becoming popular about 1830 when some people were shocked to discover that yeast was a little beastie. Combine this with the threshing machine making higher quality flour much more available and biscuits & gravy probably became popularized by the US Civil War and hardtack. Soaking the rock hard hardtack in any form of gravy would eventually make it chewable. Lobscouse or solomon grundy was a popular stew thickened with hardtack or similar ship's biscuits. It is not a stretch to make this from pemmican or other dried wartime meats to make something looking more like our sausage gravies.
Fellow veterans may have found memories of the similar "SOS" served in chow lines since at least WW II. SOS gravy was often made from chipped dried beef or hamburger over toast.