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.
re15revisited, rmn1981, Koopashell TMK #Meem, [email protected], ❁komeiji satori❁, s0246813579, Seitan, Arx The Inexperienced, TitaniumSpyBorgBear, outgoing4325, Warpath-the-drone swe, dirtpeanut, dirtpeanut, Blaze the Pegasus, Dis Dolphin Guy.., L4TF2 | The Sniper, [email protected], LazerSpy, LazerSpy, PeculiarPsychopath