After looking thru "American Gun Makers", "ANTIQUE PISTOL BOOK", "Cival War Firearms", "CONFEDERATE HANDGUNS", "CONFEDERATE ARMS", and FLAYDERMAN'S GUIDE TO ANTIQUE AMERICAN FIREARMS", I can say that no one shows any pistols built by Clements & Gibbons.
Of course that doesn't mean there never was such a company and it doesn't mean they didn't build this and other pistols.
During and after Colts production of percussion pistols there were a great number of companies who copied his work and sold them to the general public until threatened with lawsuits or until percussion pistols went out of style.
There are some mentions of the name Clements however these refer to different Clements in different locations and none of them mention a Clements actually making pistols.
The pistol in the photo is interesting although the bluing looks too good to be over 140 years old. Normally bluing that old has turned a brownish/blue called patina.
If this gun is being sold as a true antique I would be skeptical.