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.
Mose, kuigi3, andresdeleonpaniagua1997, marcsmendez, ▶ Μurasa ◀, Raccoovius Maximus, ferd_lester, ferd_lester, cristobal_2005, Sonicash, Mario, kamal_saber155, Gman010, Luggy Rewards.gg, Dazkid33, AnimeGirl, wickedimages193, IX, ƝAƤOƝƳ, Super Ratchet Limited, Yueenode, El Taquero 65