Dennis,
This gunsmith is probably George Gillespie, b. 1823 in Aberdeen, Indiana to parents Robert and Margaret Gillespie. George was working in 1860 at Rising Sun in Ohio County, Indiana [Indiana's smallest county], in deep southeastern Indiana near the Ohio River and close to Cincinatti, but not quite on the river. He was listed as a "boatman" in 1860 census and "gunsmith" in 1870. He also had two gunsmithing brothers, James Gillespie and Robert Bruce Gillespie [note the "B" middle initial]. This man fits the timeframe, the right location for the style and "Cincinnati" lock, has a gunsmith brother with the two first initials of "R.B.," and is probably your man. Jeff Jaeger of Indiana, who did the recent "Indiana Gunmakers" book, may be able to tell you more about them. Perhaps Jeff has seen their work, but he did not illustrate a Gillespie rifle in his book... and I have never seen [knowingly, at least] one of their rifles.
Post Script: Jerry Noble's compendium, "Gun Trade in America," has a listing for "Old George Gillespie" that states George was from Aberdeen, IN, and later Arcola, IL. "Signed [his guns] 'G. Gillespie' in script."
I checked the "Find a Grave" web site. There was supporting information on George's Indiana birth and Illinois death. He married Mary Ann Boyle on Sept. 4, 1850, in Ohio County, Indiana. She died and was buried there on Feb. 20, 1885, so Gillespie didn't leave Indiana until after 1885. He moved with their four grown children to Arcola in Douglas Co., Illinois, where all are recorded, and all but one died and were buried.
I checked Curt Johnson's "Illinois Gunmakers" book, and he had no Gillespie gunmaker listed... perhaps George retired from gunmaking by the time he moved to Illinois.
Shelby Gallien