IIRC, "gas operated pistols" are different than their rifle brethren. With rifles and shotguns, the gas is tapped and this pressure is used to cycle the action via a piston (AK) or directly (M-16). The "gas operated" pistols I'm familiar with Desert Eagle, H&K P9S, H&K P7, use the gas as part of the locking system.
Full power cartridges such as 9, 40, or 45, require a locked breech to protect the shooter from the pressures of the cartridge. It is necessary to allow the pressure in the chamber to drop before cycling the action. When the barrel travels backward on a Browning action, the system is actually "unlocking."
Some lesser calibers, like 380, don't require a locked breech and are more accurately defined at "blowback." Sub machine guns in real calibers are blowback as well because the bolt is so much heavier than the equivalent pistol.
"Gas operated" pistols use the gas pressure to delay the cycling of the slide until pressures drop to an acceptable level. This means that the barrel is fixed and the pistol capable of remarkable accuracy, at least the HK's.
The only drawback, at least with the P7, is that the area that traps the gas heats quickly with extended firing, meaning one has to let the gun cool occassionaly. This isn't typically an issue except in classes that fire large numbers of rounds.
There are a few exceptions to this. The one I immediately think of is the "arm gun" that was developed for pilots. It was basically a handgun, used 223 ammo, M-16 magazines, and a true gas system.
[This message has been edited by ajp3jeh (edited 11-11-2001).]