Depending on condition and originality (no modifications) and assuming the rifle is a 6.5mm Carcano, about $150.
You MAY be describing a Carcano military rifle. If so, values about $150, depending on condition.
Check the receiver and barrel
Your rifle is likely chambered in 6.5x52 Carcano. This can be veried by a gunsmith.
Unfortunately, we have no idea what rifle you have. Gardone Val Trompia is a town in Italy. It is not a make/ model of a gun. It is possible that you have a 6.5mm Carcano rifle made by Beretta, but without seeing it or knowing the markings, that cannot be determined. As a military rifle, there is no "owner's manual". However, there is information on loading and firing the 6.5 Carcano available. To use as a repeating rifle, there is a stripper clip that must be used to hold the cartridges.
No published sn data. 10-200 USD
$125
Commonly called the Carcano, this was a bolt action military rifle that fired a 6.5mm bullet. It was the standard rifle of the Italian military for many years. It was also the type of rifle reportedly used to kill President John Kennedy.
You don't give us a lot to work on- but Gardone Val Trompia was, and is, the home of Beretta. Among other firearms, they made different versions of the Italian Army rifle, the 6.5mm Carcano. Value of ANY firearm is dependent on the specific model, and condition. The Carcanos are not especially high dollar collector's guns, and may sell for $100-200. Visit the website for surplusrifle.com for some more information, and see if yours looks like the ones there. As a side note, the 6.5mm Carcano was the rifle reportedly used to assassinate Pres, John Kennedy.
Beretta
Well, we CAN tell you that it is Italian, and lkely a military rifle- since Terni was the national Armory for Italy. Beyond that, we need some description.
Your rifle is an Italian military rifle known as a Carcano. It was manufactured by Fabricca Armi Terni, the Italian military arsenal, in 1942. There are several versions, yours is probably the "fucile" or long rifle version. It should be chambered to fire the 6.5x52 mm (or "6.5 Carcano" or "6.5 Italian") round, but some fired the 7.35mm Italian round, or the 7.9 mm (8mm) German Mauser round. You can find more information at: http://personal.stevens.edu/~gliberat/carcano/
Terni was the national armory of Italy- you likely have an Italian Carcano military rifle- but we can't tell you more without more information. Sorry-