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.
ardhocool, roboteck.ao| trade.tf, ankitkala.2010, -:=БАТЬКА=:-, jwilson4431, bananaking2015, Иван, [YT] XLuke, antoneaggabi, JAMESGAMER, sebastianmartinez766, campera al habla, Colonist Yellow Malanta, mertcancolak11, okurak7, Стич, RANDOM IMPLOSIONS, Borodin, Baka3310, Intimidator 305, Wolfofdoom3, TF2MlgEngineer #TeamHeavy, Silent Cowboy, dr.d, X dude Awesome, themackboy105