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.
AmoTek♅, HG | Talbot, Player, zanofa88, antonidragonesgo, alexshulski, yamahonkawazuki, re15revisited, Saysay, Shindou Kosuke [KROME], He||oween, Russia Greatest Love Machine, MaximilianoEzeQuiel, [HG] IMI4tth3w, ☬HAZɱλτCʃરεεȘτϓιεરC, gabrielferrada, Lucifer_, desupacito