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.
Araja, 7sectoDeath, TheLankyGamer , Rex, hardtofindlucas, Lesko, Natsumi, sootymc, Fabian Fischer, Major Booti, El_Cocow, foxyGamer, carloseduardopinedal, zakarisettou2016, surrender to join islam, gilalme16, josemiguelsevillaquesada, EinfxchKadda, lucas_stella98, [GGWP]BlueMazter05, Artur Sem H, Cobra, Mistrz Yoda, godlike.exe, SwitBEE, Stinky Butt, sahingozataberk9, Niklaus97, maximilianoreyessoto, neronlopez7, thebestnsmb, Fabled, nccindy23, ReDvΔc., FunkYoDaughta, Koneko, brodokastmaxpayne, corachita2018, hernandos877, hafton92, Papi, Coringa | SDB, Slinder, emreari817, jroaks12, king.bez1123, jorge.halo100, chriskim6, badwolf0216, brunodiazwas0 +4087 others
