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.
francofuica701, Alien, fernando28_07, [LG]logicman|RadicalRP, marco-antov, WitcherRevan, rook_cj, EpsilonLombax, Toti, David, imlovechicken, HomeSlice52, carlos.hackmann.olaya, Springtrap, L_A_S_T1, Raven30789, JeffreyWoods28, JeffreyWoods28, JeffreyWoods28, JeffreyWoods28, johnnynguyen20028, Dean, DRBUDDY100, RexChiller, RexChiller, To_The_Max, To_The_Max, To_The_Max, sesilon-chileno, joakim_06, hansmartin1810, Lord Jackson of Gallifrey, Garfield A95, YamiJr