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.
Golden God, Suphax, trifaniayu, iovenko1910, DJ Kyung, HG | Webflashing, re15revisited, BLaDe:., Mr Soarin, DINNYFORST, Someone, 댕장꿍, rattle me bones, oɥɔʎsd, oɥɔʎsd, George_Payne, sayedziad, dumbaq, 바나나 스무디, King Narwhal V, Apex, sa, seize the memes of production, Woofwoof1010, Coca Cola Bear, KOR Mr.Pooh, FarhanHQ™, Jurisdiction, BoomOfTheEpic, alonso12, Sam 'Anal' Fisher, Atomic AR-15, Turd, ✘r.i.p✘, TheFrostBible, Buff Bagwell, QuickShot, jw93859, Outspokenbeef15, thewildbirdmedic, Warpath-the-drone swe, Black_Python_, Minimole the Lorax, Pepperchini Furlando, Scott, Ignis, Leafos (マック), LuLuKinkyJew, Bloxyest, ☣Pyro +6 others