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.
Kyung Marine, re15revisited, BLaDe:., Lina Inverse, Someone, rattle me bones, PUSSY HUNTER, PUSSY HUNTER, george_16, sayedziad, dumbaq, 나이트, King Narwhal IV, Apex, sa, seize the memes of production, Woofwoof1010, Coca Cola Bear, KOR Mr.Pooh, FarhanHQ™, some unfunny skeleton, BoomOfTheEpic, alonso12, Sam 'Anal' Fisher, Atomic AR-15, lemoon 3213, , I GOTTA VLOG MY LIFE, QuickShot, jw93859, Outspokenbeef15, thewildbirdmedic, Warpath-the-drone swe, Black_Python_, Minimole the Lorax, Pepperchini Furlando, Scott, Ignis, Leafos (マック), LickmySims, Bloxyest, ☣Pyro, Wallpox, Mister Lunacy, Gnome of War, PypeBomb, Orange Lucario (Razor), Just.