Nerve gas my ass.
as anyone who knows anything about chemical weapons can tell you, nerve gas makes all the muscles in the body contract violently, this means that you break your own back and eventually your heart and diaphragm stop working.
Nerve gas does however have antidotes if you inject them fast enough (like Nick Cage in "The Rock") so in theory nerve agents COULD be used (but it's too risky), however it says that the Spetznatz SHOT the Chechens, which means that they were still a viable threat, they would have been writing in pain unable to move if it was indeed a nerve gas, so there would be no need to shoot them. Also there would be more civilian deaths because the commandoes would not be able to reach them all with the antidote in time.

What they most likely used was a powerful sedative (similar to breathable anesthetic), but theoreticaly people who were closer to the source of the gas could go to sleep and never wake up (it used to happen in hospitals sometimes)