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.
LAMAR13, ReM, aladin, JWells, StokesMagee, Kiroxsu, Nagini, thesunstroms, Da Baby, Freshy, Majual, serpent925, destroyereconchannel, bourguignon549, Blue Power Ranger, captain obvious, tgman33490, Xzplicit, LobsterFTW, KillersEdge, ⎠⎞Le Potato⎠⎞, WaterMellon, KirbseY, haroldowens47, Maim, angelofdeathpony, Guybehindyou, Turbo, Mista Heita, TheSwitzCheese, The Iron Golem, Itsslinkyy_, ⇈ seasko ⇊, A2H1EY, DanWWM, Feeesh!?, Numb, dab my life into pieces, Garry mod lol, TxRx, xavdogau, Sweden Ball, Ralien, Spheritual, BonkRipper Of the CHEEKY BREEKY, simon66pb66, sirdogg, [UCG] Hippity-Actual, Johnanater, Giant +524 others