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.
tanabordee2559, [GG]Cupcake, LuigiChu, Bambie Ht, foryiizalt123, WeilolGx, heartlikedecember, [ιɢ★] Matthew ☮ ✌, ecalassa, SKY277, daredevil4u, calassa, Nixel, KyleisthecoolguyDF, Knuckotheninja, paopaoza1155, paopaoza1155, alpha_erick