have launched smokes from my M203 a few time to varying success, but even when CO2 powered it has been hit and miss as to if it goes any distance. If you want to launch pyros (flashbangs) then just buy purpose built for it, the T.A.G. Innovation grenades are awesome.
Aye that is how I've launched them but used CO2 rather than green gas. When it is cold you don't get anywhere near as good results, and throwing them is easier.