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.
Heretic, Life Decoded, neprest87, (Dual Elites), Blitz, 귀신금속, Kentucky Thy Grill, #Flu Boi, #Flu Boi, hickory_nutsack, MokouPrice, guesplane37, drexdsa, gordeev.alexey, metefen128, DoSnUeBe21, FrangoBatataDoce.exe, Ith, redBlue72, Jurisdiction, lin002100, Killbutt, Killbutt, Viper, UltimateRecon98, Serena, Serena, Serena, ghosteh, @< ><ema177>< >@, Le Chevalier Noir, フラックス