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.
Gangstergurl, my.little.pony.and.dinosaur, TheCakeWolf, PinkamenaD.P., Hierophant0, jon.sena.03, vaughan.sage111, vaughan.sage111, Mrs. Exe ❤, FouFouQ-TV, Lonnie, mr krabs WALLET, Rockguichesse, Ｋａｙｘｙ, Mr.Fire, Kaliba 69, Warpath-the-drone swe, fawfo