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.
mehanikmoduw, josedavidcs02, |LaG| Sherman, j00x7, paskalito13, Sh4dowV0rtex, RedHunter, jhernandez.2000x, aa8326138, nagaprabhukatta, josedavidcs02, josedavidcs02, DMessham, DankDestroyer, aminehajjem2002, Neji_Sama127, mahmo.s.121, doudoumanix76, scotymathieu, Phoenix_Talon, CrimsonCrafted, rpguileb, freddy.kacper, Javik12, allanculham, juggmax, leolove055, t443325, nnabilchiban01, blackopsfast5, qwaryes, silinc.sebastian, da1337noskope, bentadil610, Guilherme_Senem, groupks9, bflmn, DrFlare10, That's AreniX! [mS], wasker12391, It was but a walruse!, alessio124, valeriusmagnus225, fazafreeman, Blitzy Blue, wayne11440205, maximilianoabichayn, nabilyusof, sshahrabadi92, Cacti Inside Your Urethra +97 others