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.
to4187yama, V-v-Raven-v-V, t443325, joucajouca, QTG坑, ❥Lun4tic, takamanokagura, mokomoko06352, tonton8989, white2k | Japan gamer, mls, minakomi0, Trager, 他斗, MAXIMAj11, canihaveaworld, Mr.Pootster, Sanic, xUPYAx, patakapata, patakapata, HESHEЯ, GOOREMU, rhadz1234, thirsha4242, DrMarvosa