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.
misu_misu40, Avetho, stricklanderic24, monster21cz, danielrakus1, rcvsj2004, nabilyusof, jakeboyharris, Will, Zenocidi, hacking black mesa gamer, axelito42, Tryingdieing, [iridocyclitis] Ty Wab's/Dave, ✔ │MuffinBoss© │ ✔, [GG]Tense!