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.
DJsakara01, s-mukataev, anhducemhung, kleopatra_kadir_7878, mj01032003.mj, igorcurak, ShadowKnight115, zennat.3195, mushraf_2003, Gokan_Nagara, NerToxPvP, Nopity, anilcgart, guahilal, xXManspiderXx, jessus3588, G-MON, OxzeN_YT, N4threzim, Baconmaker, Grievous, Confused Silent Hero Link, Googly Goobly Cat, RyanTheNerd, FalubiiCZ, wzm0630wzm, îÑ§áñêÇℛⒶℤÿ₁₀, sim.ringwelski, Chompskie, Xen_12, Sans, l.maire c, The0nlyGH0ST, isaacjackmon1minecraft, Alt, DrakeAcheron, CLawler, Kerby, The Unlucky Medic, ✯ XxXxFrostyXxXx ✯, philpenrodriguez