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.