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.
dbrett238, Xenogelion, xXx_Karabela_xXx, Slaurtis, Ametis, DABOMB, falseprophet141, whiteshark01, KursedVixen, (Tal'Darim)Havoc, hs10662, Sneaky Snake, Mir023, Horstron66 Brown Spartan, 플포, ObiJuan, RAWRsoft, HUN_ManDark__HUN, Zachariah, olkyt, volume1539, josedslkfjdgsfkjpdjffdljk+, Very Spoopy Engineered, χχєση | кαѕυ, proli2000, AcidicPone, Frost1551, єℓɑиɨs, 돼지[pig], lonely19682005, Chronos, DeltaForce236, Dreadmaster231 - [Offline], Dreadmaster231 - [Offline], uberchain, LiquidFire, R U Ready?, 메딕, [email protected]