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.
Big_Boss☭, \/endetta, dogukan_aksoy222, cheschess, (ZERKI), 심정우, [24th] GammaGamming, andiew2548, FPSgenerator2.0, pokernightpros, Mbah_N4k4l, qw,f12452342590=55$%^&*2k19=, TheRandomKiyru |AI|, Eddie Dollins[Barnes], oldE_, Xeximus, Atomic Tacoz, DeX, ddhs5845, pascudarius81, Blons Monkey, Deko desu, steelwork, Wulfy, Patrick, Patrick, Thal Verscholen #TeamHeavy, Thal Verscholen #TeamHeavy, wildchee, 감귤 작작먹어, 감귤 작작먹어, TheSzabiHUN