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.
Chang3d, [BG:O]FreddyPlaysYT, Darkdergute, Lucy, AlexManiac, Raxshi, James J, [PG|Owner]Pckool, Owzmo, Leon | Sharesocial.ga, JavaNava2, Mr Panda, FiveSeven, ToXiiC, martin.drelich1999, FuRyX, [ProjectLife] GEØRGED11, [ProjectLife] GEØRGED11, [ProjectLife] GEØRGED11, [ProjectLife] GEØRGED11, [FR] || Boris ||, Бен Ладен*BenQ*-CAPITAN, TheAct1on, |-ICE-|Shy Guy, [IM] Nobody., Leod, kk, BoneZed, ☆ HazardSkii ☆ ( ͡° ͜ʖ ͡°), [RDF]Aekola, Lumii