This download looks like a map. You need to place it in your maps 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.
LastZlo, re15revisited, minecraft55.teo, im a melon, Melon Le Prince, Rick 'Knives' Hendricks, Truck, KLAWZERAZ, Zarous magicdrop.ru, nongteam_seka, nongteam_seka, Victorious, Badmiral Ballen, Actual Dweeb