This download looks like a lua file. You need to place it in your lua folder inside the Garry's Mod 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.
Wippy, -:=БАТЬКА=:-, Drew SlxyZ, TheIronGaming FNAF Endoskeleton, HesNotorious, Flash™ - FutureRP.co, Loading Idea..., I Я ДШЁSФMЭ, Alarm, lukinhb, lukinhb, Johnny Whinkel, Jurisdiction, kirbygame126, Mr.Gravey, Ar.Lam o_O, ҉;GBSFM ҉;, wheresmygui, Puggilicious, Sokkah Abdelkader, Sokkah Abdelkader