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.
Signature, escolargh360, Greil, Comrade McNuggets, ciddiyim., Thick Thighs ʕ✿╹ᴥ╹ʔ, SpamLine, SpamLine, [SR] Vix / Jayarr / Doom [A], Schokoriegel, Ruky 7w7, Gryphin, Szym_TO, The CycJIuk, B055M4NN, Hatty, juniorx199709, kaikejcd745, JJXN, ArcaneShade, nafizchawdhury56, Brave Cyphra, Eerozx, koolkat, Aramadon, Fuzzy Unsure, nickmaguda1, Gallows, Lizard people!, charly232815, kaikejcd44, kaikejcd44, kaikejcd44, MergasTroll, MergasTroll, Classic-Pony ツ hellcase.com, Itian, Golf | UndergroundNetworks, MarcelHD40, Twilight Sparkle, KissfromFrance, PhasorBlack™, Albert_WeskerT, tevtelka, Abnormity, Aurora, ¨vesq_, ArtemisTNT hellcase.com, urakozachok, Sergeant Spaghetti +1109 others