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.
josseph1256, bagastribismasaleh, Al3x30 Skinodds.com, yazansbouh, cakrabayub.r, [Springy]Nightmare Fredbears, davelikadja32, gundamwxyz, sniper554, TheChillingShadow, isabelcarruella, drexdsa, , 6077, el real player ecotize, Miia's S.W.A.T. Team, rage_unturned_14, cyrusalim31, boxvader, Fondateur | Kriax