This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons folder. You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • addons
          • darkstern <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

• darkstern/
  • assets/
    • css/
      • style.css
    • images/
      • background.jpg
      • home-icon.png
      • icon-bg.png
      • logo.png
      • map-icon.png
      • screen-icon.png
    • scripts/
      • jquery.js
      • main.js
  • index.php
  • news.php
  • README.txt
  • settings.php

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