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.
[ME]chuckjoens11, iconebg, RireTram, Dunk1n0nDeezH03Z, SuBDivisions23, JEFF, |RG| Blank, BillversTheSecond, DiRT071, krafftd, ThatGuyKush ( ͡° ͜ʖ ͡°), QuasarMarsthe97th, |e|, traptboii, Krzychu 140, KFS Hyze Merc, Spintires, Razorroboham, Bwamo, Uros Uki, Rinald_SVK