PS! Since latest garrysmod update to "Steam Pipe" format the ADDONS folder has moved! All addons MUST be moved there!|
The new location of the addons folder is "Steam\steamapps\common\GarrysMod\garrysmod\addons"
Extract downloaded files and place the folder (Ex. "CSS Content Addon" folder) into "/garrysmod/addons". Enjoy!