To install manually, go to: DIR\Steam\steamapps\common\GarrysMod\garrysmod\addons\
and now all you have to do is drag and drop this GitHub folder into their. In the end, it should look something like: DIR\Steam\steamapps\common\GarrysMod\garrysmod\addons\scpcb-snpcs-master\
or something like that. I'm not downloading this entire file to see the name.