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.
That Guy Right There, Tails_Doll_the_Fox, G Froman, Витой, OSITO (TED), caav56, soundfall, 777krasav4ik777, Alien, spaceattack11, tiptopspin, nnn0611, MisterAlpha, mccmcc123, *****ed, Ack Ack, khushsikka7, khushsikka7, rmn1981, Steadart, The Medicine Peddler, Iris, BayPig (lego cunt), judge judy, judge judy, Mogabi, demonsoul, farael-maia12, (FLoFF) Megaraptor, volume1539, your Father, resistor58, HALFLIF3THR33, UNFORFOZIS, cassiokid10, C++17, BLACKM3SA, Dopey, ☢AlieN☢, MonarchMonk, The0nlyGH0ST, AWACS Space Eeyore, The Era of Plem Plem, Arctic0ne, swedes, low budget flights, //AV//Anticitizen_one, Xoma-X4, Martyr, ☢Gladius☢ +1 others