Adam when they talk about gas powered it is not referring to the gun powder, it is referring to the way the gun unloads and load incoming rounds. They are referring to the mechanical action of the gun. Guns like a western six-gun revolver operate by a mechanical connection that rotates the cylinder, when you cock back the hammer. A shotgun with a pump mechanically ejects the empty shell when pulled back, and advanced the new shell into the firing chamber, when thrust forwards. Old cap and balls are mechanically loaded by hand with a long rod and hand packed to load. A lever action western rifle, also mechanically ejects the old shell and advances the new one into the chamber. Bolt actions like big game hunting rifles with scopes, eject and advance the new round with a sliding bolt. There are many many hand and rifle designs using "hand mechanical means" to eject fired shells and load unfired shells, hence the term "repeating", holding multiple rounds, rather than single shots, but by some mechanical device can reload rapidly.
In gas operated guns, they utilize the expanding gas produced by the bullet that just fired, that is a high pressure pushing the projectile out of the barrel. These are called automatics, or semi-automatics. The machine gun is fully automatic, when you pull the trigger it just keep firing. Semi-automatics are one shot for each trigger pull. In either case, the high pressure gas, as the bullet travels down the barrel leaving expands into a internal chamber, that slides at mechanism ejecting the fired shell casing (the brass), and springs advance the incoming shell to the chamber and return the slide to its neutral position, locking and loading the new incoming round. It is very fast, all done faster than you can pull the trigger, almost instantly as the bullet has left the tip of the barrel its done. Some of the guns that are gas operated are most modern miliary rifles, the M-1, M1-carbine from WW2 and Korea, the M-14 and M-16 from Viet Nam era, etc. Gas operated guns have been around since before 1900, the military 45 is one of them, The German Luger you mentioned, plus many many more. Many bird hunting or skeet shooting shotguns are gas operated as well. Many plinking 22's like the semi automatic ruger 1022. All the cops have converted to semi-automatic Glocks with many rounds over the old 38 special revolvers, now probably useless. Gas operated allows you to really lay down some serious firepower with devestating results.
In other words, they are a lot of fun to shoot plinking. You can just blast the hell out of anything to your hearts content. All boys love firecrackers, and automatics are the ultimate in firecrackers.
But it costs money everytime you pull that trigger, bullets arn't cheap.
Hope that explains gas operated a little