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.
Blodreina, RampantLurker, xmarin43, , [BLN] Kaling, Lt. Dan, Lutrix, xB4CKUPx, Wess, XEAR, partyr0ckking100, Demon, FlabPack, FlabPack, Reno, Lieutenant Macaroon, kridens, Colby, Aleph Le Borgne., Salt, nearrrrnearrrrr, boomsta99, ZeroCMB, Moey, Eshay Magpie, Xaron, ~u~SageYTGaming~p~, omega TPG, "Rain", Ralle, Paul Parker From Nelligan, Brinkleys (UG), DaddyPippin, Gabby Mageestery, [AG]r3azy, [SLRP] V4pale, Longso, isutov.02, [SOG] Ginkgo, Sebevedomy, XxClankyxX, marshalllane7, harryborthwick, paradox, Zayna, Instant, ColosusKnight, Toxer, James, robin.gangl62 +3607 others
