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.
beq92673, Cetex, , Markitos[KG], Dimkin™, jax, Atropus, laut, soncraftkingcraft45, ✅MarnOx hellcase.com, Azubbal (Elvin KOV), Vanilla Soft Scoop, pinejack, James Positive, Ulrich, RyuZU ~ Akaya, [NGN] BossDarkReaper, Alice Angel, , [EVORP] Death, |EBG| Brandino