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.
[RTFM] Evilflora, Gaminghunter, Llixo, Stifler ✯ ΔΨB, Sergio232, ◄۞Omnipotent۞►, |UDG| Vergenusswurzel, Smitefire♛, Ma7et, Carnate, Jack Mi Hoff, Ryan, Ryan, Princess Melon, ドラちやん, Marmiteiro, Keshwoof, Koyou, Koyou, Koyou