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.
ddpetya, HaychQ, Dotheboogey678, sanchezversolattoaxelsadoc, EwanBoii2, JJ12, favcal, Xaryonx, Higuain Sniper, goos-long, zottellotte, gamehag.com: IcySpikes, thuxys, Mateaso24, com.antilles, Chud, Venum117, bebreakey, nongtiebk0004, jose.veloza, tstbaker33, paul.m.w, ivan429035, AY| SHADOW C4T, VITAL, sgpt12, DoOmMaRiNe, hassanrubbani2006, (_BLYATMAN_), shrekatar, RexxlxD55, sans123lol, Cha0s (A.K.A TheSaltMine), orion♪, Krt3k, Asriel the Raboot, ivan4290, Zekan, Salad Fingers, gregorscepanovic, jerri_ld, Sonic_Extreme, i want not live, s.hizli, owcz.Key-Drop.pl, nathanland2018, the heavy weapons guy, mamedeuntar, secretmanstudios, ademnesirov3 +1024 others