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.
tiptopspin, nnn0611, MisterAlpha, mccmcc123, *****ed, Ack Ack, khushsikka7, rmn1981, Steadart, The Medicine Peddler, BayPig (lego cunt), judge judy, judge judy, demonsoul, farael-maia12, (FLoFF) Megaraptor, volume1539, your Father, HALFLIF3THR33, UNFORFOZIS, cassiokid10, DragonStudio, BLACKM3SA, Pepsee, ☢AlieN☢, MonarchMonk, The0nlyGH0ST, Nyoom Nyoom Nugget, The Era of Plem Plem, swedes, G-Boy, //AV//Anticitizen_one, Xoma-X4, Martyr, ☢Gladius☢