This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod folder.

You should end up with a directory structure similar to below:

• steamapps
  • common
    • garrysmod
      • garrysmod
        • lua
          • autorun
            • ghostbuster wepons/
              • info.txt
              • ghostbuster's.lua
              • lua/
                • autorun/
                  • ghostbuster wepons
            • README.txt
            • photos/
              • 475fdf430257c3a607f36a97658fa367.jpg
              • cc71fc7518cfd3ff24e155d6518c143a.jpg
              • p.jpg
            • ghostbusters main song

• ghostbuster wepons/
  • info.txt
  • ghostbuster's.lua
  • lua/
    • autorun/
      • ghostbuster wepons
• README.txt
• photos/
  • 475fdf430257c3a607f36a97658fa367.jpg
  • cc71fc7518cfd3ff24e155d6518c143a.jpg
  • p.jpg
• ghostbusters main song

03kj20f03232, shiamirre, loganjj3015, Dozeplaz, unclebody, optimusmikee, Redscream, D3f41t, danilo.majkich, ryanpeterson933, artak.sargsyan95, Pidän Leipä, MARIO44422, moonprincess2010, BooBeep