The GSMG is a gas powered submachine gun that fires .177 or .22 BB pellets.
The gun shoots BBs at 600 fps. If it was able to achieve this with even the heaviest .22 BBs it would only generate just over 12 ft/lbs of energy. Before you get too excited about defending your castle with this legal-to-own-without-paperwork machine gun, it is defiantly not a toy but is not nearly powerful enough for self defense. The GSMG is currently a prototype and the final product will sell for around US$350.
I may be remembering this wrong, but I think after WWII the Allies discovered that a German bicycle-maker had build an air powered submachine gun and planned to use it as an insurgent weapon. More recently an infamous British gun activist designed a air powered submachine for self-defense (he later went to prison for his illegal collection of homemade machine guns).
UPDATE: I was wrong, it was a French bicycle-maker and it was not a submachine gun. See spudfiles comments.