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.
prokulevich11, ☠D̴̈́ò̶͈c̵t̵̀ͅo̸r☠, 1, 2241586939, ThrustAttic335, BLOX, seemeemail1, kirillikin123, joseyegen1995, ruslan.ore1999, Bock Buster, fluffyw0lf, Stressed Operator, K0D4, zanofa88, Walter Utility Jr., MOberKing, archicastor1, GENTLEMEN SPY[ASİL TR], Waiscrim Gaming, MindGate, santi.damico, Stalker 112, [SERINITY] DrayZor, ExrizE Get-Skins.com, Braindawgz, tizubogdan, tizubogdan, RiP, MuhammadIzzan_fr_, easycat, Jenkee Reemer, Jenkee Reemer, flo, TOO-BEE!!, ★λDUM JΔN$ON★, kenny_iscool, juanlima417, djokersted, Wicked, robbed by a chicken, robbed by a chicken, robbed by a chicken, Lasse Bengtsson, Ful76, Darknight9818, PapazKaçtı, Daniel.MediaFire, Daniel.MediaFire, Mister Blatant Apathy +32 others