This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
sanyipapp, komarchevav, FuzionNitrox1, The13th Jason, cute.boyammar, FredBed5000, eleiber, Ḟwêêdoḿ ♥, adamir2004, dominik.c2006, haider144, fearlesskdog, fearlesskdog, snapthefox52, PL Comeback is real, K4ZP41972, WAVECZE, leklo375, (음성) SCP-173, Rose, linuxser1234, wildark66, [QC] DarTax, rodelbenedictmonte, Thundray, QUISAMBER, bonniesemface1.5, MatijaTheEva, amottie, Wero, alirezabakhshi2001, norecess333, evandermarvin, Nykozeh, Kyle, Zabujca, Exotic_Evan_Fong, scammedperson, ananabf, Sibylline, kezarix455, klmpackaging, dinamiccreator, Warrior Noob, Ruzz_Intent, Here's Johnny <3, Spectre, MingeBag, nedukas211, Martinvenegas333 +95 others