French guns in general simply don't have the "brand value" of British guns, with the possible exception of makers of "best guns" only--like Granger. Like Montana said, Guyot turned out some very nice guns. There's a 16ga on Cabela's website that I'd describe as pretty much a "boxlock best". But generally speaking, with French guns, the mantra of "buy the gun, not the name" is excellent advice--although Guyot is a good name, and it would certainly add value in comparison to a gun of equal quality and condition with either no name (not uncommon on French guns) or a name that no one recognizes.
