To install GSMOD you need to:
1.Go to Steam, now to Library and search Source SDK Base 2013 Singleplayer and install it.
2.When it finishes installing, Download the mod and then extract the .7z file with 7zip or WinRAR, rename the extracted folder to 'greensoupmod" and copy it to "C:\Program Files (x86)\Steam\steamapps\sourcemods".
3.Now restart Steam and the mod should appear on your Library, enjoy!
Note: in Source SDK Base 2013 on steam, you need to press the three dots button and go to properties, now go to betas, and in beta participation put "upcoming" for the game to be able to run.