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.
Fx_Gektor, lmpzy, the.great.phitus, jackkeosodsay, Nagibator3000, nightrainmusic, Reaper, Gold[Anti-NSS] Faze, Perro_Verde, _SamuraiJack_, ctorreslazo23, Justas, Shaq, Wolfpack Heavy, Nathan Drake, ^RussianTiger^|Hadouken!|, DKUD1337, sugarblood