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.
davidwerner007, babaarda2005, ATM, adityasurya1434, Deaf Kikbow, Scourge, [G] Bingegamer, [G] Bingegamer, radin-f, suradechatarinant, sandiegoklassik, HailBolbi, Tricky, Mr.lemon☃❄☃, ggggg, ✯ Shark, [RFAI] La Juene, illuminati, M_Farhan_A, Orcssasin, gs_umutcan2011, wildchee, kyle.stokes321, Uros Uki, 감귤 작작먹어, sumeetgupta501