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.
nongaorapin, Mantequilla, NekoTrans, herod4833, borakognjen762, JaiderSv, RedbearPL key-drop.pl, cristherabbit123, RobloxnoobXD PL, zidaneelefante, NCS NIKE, Key-Drop.pl, agustinavila24, pakgametvxxx518, aliardabeyaz09, Ms Draca, {15th HI} Sdt. Jessica, Tonayan13, Lach, DrajeBooM, Женя, hohoho121121, stas_terexow_2, 95k, Shadow_Moon, meinhard-blutengel, mybaconuseristhis, bossza_2005, zockenistmeinding123, Sir Smashington, giulianno05, AnonymousAnime and SunlessRp, lol1234567890zz, lol1234567890zz, giulianno05, Nicosh Juega, Creed Nation, wualitoelproxdd, simonparraguez1, MaxDiWay, andriu229, tungdylan, LiriunTheCat, Yy}|{ou, josueyeyelessjack, Hunter_Kyro, LYON S.P.D, itsbyluke, ŁR1RRoFFicial$, TuBernarditoxBnOjigins +390 others
