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.
Ganerdene_od, hgdgffggfgd, Tober, EbizoC3PO, nonutsforyouall, Saysay, Kenny, LordSpace109, REKTIFIED, dusan.mitrovic004, paja.reljic, mjh9994, swagalishes00, batugoztepe, GojiFan1995xyste, Taste The Rainbow Bitch, Knuttman, LegijaJSO, MythicalZoo, kiztak