get steamcmd https://steamcdn-a.akamaihd.net/client/installer/steamcmd.zip
place it in a folder that is not your downloads folder lol
download it and run it

type this in

login anonymous
app_update 232330 -validate

wait for it to install



what other guides did is extract it using GCFScape or smth then place it on addons folder, but i don't really recommend doing it since it can probably replace base gmod files

instead you open up mountcfg in Steam\steamapps\common\GarrysMod\garrysmod\cfg
and then
find the folder where you downloaded the css content which is in the
C:\the_folder_where_you_placed_your_steamcmd\steamapps\common
folder
then copy the file path of the csstrike folder then add it to your mount.cfg



there now you can have css content without breaking base gmod
and it also mounts it in hammer

this can be done with any mountable game that has a dedicated server like day of defeat source DOD:S , Black Mesa , day of infamy DOI , hl2:dm , l4d2 , alien swarm , nmrih , insurgency 2014 etc you just need to find their ids but..

it will not work on single player games that dont have dedicated server like half life source , 2 , its episodes and lost coast , portal 1 and 2 , ( you need to own it to able to download it in steamcmd )

unless you ask someone their installation with vpk files not extracted then it can work

https://developer.valvesoftware.com/wiki/SteamCMD#Downloading_SteamCMD

https://wiki.facepunch.com/gmod/Mounting_Content_on_a_Dedicated_Server

https://developer.valvesoftware.com/wiki/Dedicated_Servers_List

https://steamdb.info/search/?a=app&q=server

https://sp.zhabite.com/sharedfiles/filedetails/?id=576400029&searchtext=cs%3As