zombie riot CS:S World at war by Natasha Volkova

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
          • <-- Addon folder!
            • e.g.
            • lua
            • maps
            • materials
            • models
            • etc

paimonamber6, nav404040, Nikto.lol, Bubbafrog, adrianhaar115, npekaric1, 挽风, nhatcuong8a4, kohut0802, KINGleader, eilia_plusm, stormi, saintermc, CBA, awesome.sebus21, LuYe_26, Skefil☆, ✙Фрості, V92