If you can't find gmod in the common folder, try putting the download in C:\Program Files (x86)\Steam\steamapps\YOURSTEAMNAME\garrysmod\garrysmod\addons. If that doesn't work try reinstalling gmod and TA-DA! You will now have a common folder. Move the maps to the common folder as well if you don't want to re download everything!
If you download from uploads just download both files extract both and merge them
Note: Users with addblock may need to disable it in order to use the adf.ly download links
Have gmod but not Counter Strike Source? Need the Textures? Here I show you how to get them, for free. No more Pink and Black Squares! Some people asked about the textures on Jish's basics Tutorial, so here I help.
Where to save the contents of CSS content addon: C:\Program Files (x86)\Steam\steamapps\COMMON\garrysmod\garrysmod\addons Or For Mac (needs verification): ~/Library/Application Support/Steam/SteamApps/common/garrysmod/garrysmod/addons