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.
Lūcifer, Kyruxst, Saraevich, re15revisited, sebastianestrelladiaz, DECOY panda1, DECOY panda1, damian.nowis, Captain 3Tears, Детка геймир, YeahDoodCzLp, johnrbedwell, x][m, koosh putin(SLO), Baron Sludge of Nova Prospekt