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.
JOT.PE ARMY 3NDER, yosef.mosel2004, [KOR] DAEWOO, dexterM, Doom213, Doom213, Suicide_squadO_O, saklik987, danika204242305, irenethour_15, Acnologia438, Sal Vulcano, paoza1155, r3elity2, cpl_green, gz-59, life180way, emircamci3939, keithkenn, abdulazizbirhanu, chandlercoates55, fajrang, Grizzly, =TintedGaming=, Im Still A Noobie, 2nd DDesigns, louboyle134, iustindavid22, krisztian68, nassim89