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.
famlovato, kittyboxer1, GusQF, christopherchris004, ardayt42, dogexo, Rick C-137, tatatanato, tatatanato, tatatanato, save5431, ItsEleozz | hellcase.com, nikinson, riky86, oefe3034, TouristicFiber, TouristicFiber, gotsuliak2315, Doogls, Hello Bendy, Maru Maru ><, Guerrero96, wolves, wolves, wolves, < Voody >, TheWitcher, The_Amputators[blw.tf], hayden0324, MelonTheGreat, NightWing101, Hamyachila, ✪Аntоn, leeliqian123, [ULTOR]┌∩┐(◣_◢)┌∩┐, bulldozer.ivanovich, Undo, Tracy, TheShoeFam, Red_Bob, beastgamer555, rkrdios, 767313265, Christian, kyle555, Ray, abdoshi2010, turtleblue, frankstrade, Oskioo #2k18 +381 others