Could this be Rohm pistol (note, please, that I use the words "Rohm" and "pistol" in the same sentence cautiously).
The GM is sometimes used in place of the German "GMBH", which is their equivalent of our "Corp".
If this is in fact a Rohm firearm I suggest wrapping it back up and sticking it back in the rafters - that's the safest place for it.
A friend of mine has a RG-10 (.22 short revolver) that I cleaned and test fired for her. It shoots about a foot high at ten feet
All the best,