What is a Gardone?

In the 1858 steel pistol add, it was mentioned that the pistol was possibly an early Gardone.

What is a Gardone? Is that a manufacturer in Italy?

It’s a city in Italy.

Gardone, Italy is the location of one of Italy's Proof Houses. It is also the location of the Pedersoli gun company.

Uberti is (or was) also located in Gardone and began production as Gregorelli and Uberti

That guns proof date is 1972, stamp under barrel is DGG in a circle indicating it's an Armi San Paolo, from what I can tell would make it from a period when ASP was in Gardone Italy.

Gardone is a valley and a municipality in Northern Italy, between Venice and Milan, centered around lake Garda, where there are a plethora of Italian gun makers located, including Beretta. Many of those gunmakers were/are involved in the reproduction of American cap and ball revolvers of the Civil War period and other period muzzleloading arms.

It is where Sergio Leone and Alberto Grimaldi went to find guns for their "Spaghetti" westerns, like The Good, The Bad, and the Ugly

Early American pioneers in the revival of black powder weapons, like Val Forgette of Navy Arms, and Turner Kirkland of Dixie Gun Works went there to have reproductions made when the muzzleloading craze took hold in the 60's and 70's driven by the centennial of the War Between the States and bicentennial of the American War of Independence.