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.
Psy, Bunneh, Xeno, [TEST]SmallDisturbedChild, Cordobe, Blancoster( ͡° ͜-- ͡°), KACEKKKKKKKKKKKKKKKKKKKKKKKKKKKK, UnknownOne, carneficinatimoteo, WirAuch (Robin), Yanko, Vappy Cabbit, Giygas -- goldclaimers.net, kkoch986, Maximus Prime, 89Mendo89, [KOR]혼살, Archie, yousef.majed.s, blacktiger2013, GG wrathion, (N)-(T), CookieSpooks, -(P)- kektus, Kat, Kat, vitaliy.derezer, O'Nesix