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.
Axundov2005, klasm, frezeer0021, wolfburgt, zennat.3195, activatorulvoltzengan, pro125gamerbg, dexter, ssh99o927, Wormboy, ghibusi_david, Harshey ᵔᵜᵔ, loupgarou_e666, 38a, Silence, Rizal Borisman, YeahDoodCzLp, Sylvester's GPU IS FIXXED, [BBG] Jerkacompa, boblealeonard