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.
fhaed510, josito_7u7, esam.alhoni2015, migueltoro1128, kamiloespejopianista2010, kevinmascenciob, jjchelo665, INF | Freenky, El_dipsy, s0ywaze_, Banana, MAZN AR, danilaberk57, jeashwd12, alfonso.cabezas.celr, yadielrobert48, TH3M4R10YT, احا, PatoXO, zairvargas7, aljubooria9, ghbudini, Charles Mudfi, eme.chucho, joshuatuberjiji, superjeffreyliu, oscar, MrkrissALV, marco.don, shorunic, mateogomezce, coredasantigas91, cristopher5342, jeromgonzalezasaro, Elma-rica, blara797, piolaxdxd17, DEKU_0W0, maikelorrala, The_One, Zerouh1DR, c34527319, jchavezdgz, PacWizzard, lalo62514, jjmmeevv, jonbob04800, Diona, charlesxavier3719, m60 +8011 others