Hi, Lightsped,
WARNING! Check or have someone who knows old guns check to be sure the gun is not loaded and to unload it if it is!!
In the old days, loading muzzle loaders was a slow process, and many farmers kept a shotgun loaded in case they heard a fox at the chicken coop. The only thing required was slipping caps on the nipples (those conical parts under the hammers) to go after Mr. Fox. So, make sure the gun is not loaded. Now for other issues.
There were several gunmakers named Fisher in that period, but I cannot find a G. Fisher.
The gun is a percussion shotgun, with Damascus barrels, which clearly show in the excellent photo.
Damascus barrels were made by wrapping pieces of red hot iron and steel around an iron rod and welding them into a cylinder by hammering them together. The result was not a very strong barrel, but it was used a lot before the capability to drill deep holes in solid iron or steel existed.
In addition, the gun is not in the best of shape; it appears to be rusty and the top rib is coming loose. So, obviously, I highly recommend that the gun not be fired.
It probably dates from around 1860. It will make a nice wall hanger. The monetary value is, unfortunately, quite low. There is little collector interest in what are effectively "no name" guns, and of course it has no value as a shooter. I would estimate a value of $200 or less as an antique. Its sentimental value as an heirloom is much greater than that.
Jim