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.
BONZIBROWN48, idreznor, Look at phone, NEIN, cubic_control, rafaelprojailbreak1996, JaimeAlejandrof, YTZestyStyl, Redd Jankowski, STRIKER, jpstirad, 卡卡超卡住, Corrosion, 1111111, Kaythas, Gear2ndGandalf, candyploy, Sindri, Harold Keegan, DaFrame4U, bbenjamin1702, Askalpone, terminato012, origio, Shizofrenik, carlolelli53, StraY, Holt | Sarah <3, hamzawwe619, hamzawwe619, CaptainRumBarrels, Claire Redfield, decadent_uk100, HUNTER00128, lligranato, samuelling97, vipmomoleo, yes man, RonInFusion, Grozger Zasrogtiveslin, Spectre - U.B.C.S., Ingested, DoyleGasai, Sabun, djwildchild, Captain Chunk!, Hades, I Swore She Was 20, FrightNight, Daveo Dinkum +29 others