This download looks like a save. You need to place it in your saves folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below:
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
SAY_Sayori, ismil_1977, wolverine041269, mathys271705, famousroux, Toast, ryder22001, nikita.belobrov15, warfacegame, zach2.barclay, Anthony, jack_5