How to decrease and Increase Sound of chosen App?
OS X doesn't have a global manager for volumes of specific apps unlike Windows. Rather, you can change the volume of apps inside the app themselves.

For example, iTunes has a volume slider inside the app that lets you change the volume of iTunes, and whilst I'm not familiar with  the options menu of CS:GO, I presume it has a slider for volume. Use these controls for changing the volume and script them if necessary using AppleScript or equivalent.