DISTRACTION CAMERA
Whenever you see some enemies gathered, throw a Distraction Camera so
that they will not notice it. After you have control over controlling
the camera, continuously press the distracting whistle noise. The
guards will come over, but if not all the guards are there that are in
fairly close distance with the camera, then keep making the
distracting noise. Once some of the guards are over there, release the
gas. Sometimes the guards may just cough a few times, but not get
knocked on conscious.
                 
GAS GRENADE
This is an equivalent of the gas that is in the Distraction Camera.
However, this has a taller and wider gas range. Also, instead of
spraying in one direction, this is like a gas cloud. Whenever you see
a group of enemies, you will want to ricochet these off of a wall and
get it so that it's positioned near the enemies. It will burst open
and gas will fly out. Again, the enemies may not be knocked on
conscious. Usually, depending on how close you throw it to the
enemies, the enemies will immediately be alerted.