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.
mjalbers1969, RED보루, Сергей Пахомов, pwn3dbg, Master Pleb Lord Charger, Stanky, 평생, Tutle, PENGYU, Steven the Scotsman, idanyazdi195, [creeper]YTRedcreeper, omarabaza, MLGR3ktman69, NJPD, Matsuko94[FR], Jean Claude Vac Ban, 두유, dav.uher, 흑천, Gaminghunter, bighippo131, 특수부대 인호, sergey42.bush, Ivanandresmorales, 난마스터해된다, ドゾニー GINTAMA, He||oween, Tremble (back) D:, Tremble (back) D:, Jebus Chytrus, 코우사카 호노카, 현아갓, 미스틸테인ᕕ( ᐛ )ᕗ, Aatos Nirav, Gold Celery, loulou723, 대묘송 I 서버ON, Parsemesan, ฺ, Skeezr G.F.B.A, Nicko, Lelik, ace7418, lord of empire, [UDG]SÜRIK, MoBiDiK, MoBiDiK, Radikal, Noah +44 others