It's exactly the same as installing it on a server.
Download the addons from the GitHub repository, and drop them in the respective folder for your OS.
Windows:
C:\Program Files (x86)\Steam\steamapps\common\GarrysMod\garrysmod\addons\
OS X:
~/Library/Application Support/Steam/steamapps/common/GarrysMod/garrysmod/addons/
Linux:
*not sure*