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.
imddeas, qriC, SkyKing404, bob, PunyHappyFace, PunyHappyFace, beholdedpeasent | trade.tf, chelsen_2405, M3nTos, blas_hitos, Skullfactory123, Punisher ͭ ͬ, DELUXEGOLDSH33P | trade.tf, Lieutenant Flea, ecclesiastes217, ayyitsaj, magikxcreeper, $noselfcollisions, ▼, zErni, zErni, LordChibiHalo, Leek Master, Nopeful, Ugi04, Mr. Trilogy, Yoloasscrack, Angelfinn12, FiLzO, FiLzO, FiLzO, Small Fungi, Vincenzo Legge, Vincenzo Legge, Vincenzo Legge, Vincenzo Legge, Vincenzo Legge, ogie, Young Pappy, zooi