This method works the same for both CS:S and TF2.
- Go to the control panel, select your server, then go down to "mod manager"
- Once there, click install on whichever content packs you need installed - you can leave the page once it says it's installing
- Next, go to "file management", then navigate through to /garrysmod/cfg and open mount.cfg
- When in the mount config, change the location of "cstrike" (or "tf" for tf2 content) to "/home/container/cstrike" (or "/home/container/tf" for tf2) and "save file"
- Go back to the cstrike (or tf) folder and ensure the zip has extracted (if it hasn't, give it some more time)
- Once it has, you can go back to the console and restart your server
That's it! Your content pack should be installing / installed.