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.
UnderTale, [Fonda DeadCraft] Damso, Messiah93, -̗̀MARḰ̖-, truepick, Kira Lebroque, Kira Lebroque, Kira Lebroque, Kira Lebroque, Sir Kitty, [x24/7x]\MagGriffe/, Adventure Time, [email protected], ★ G҉A҉R҉A҉ .ⒺⓈ ★, Eaglishee, Salty Bloody Fail, oxion, Norwin, Sold you for RP, Mr.KTO-TO, Col. John Sheppard, TheBOSCH, X_RenFire_X, CrazyDog, |★_ƒοмά_★|, AppleJeb, Piehthyte, Kekly, iDraKz | Eryzia.com, SonySeva, Satrum, Ice, , Skate-ВС РФ, Enzo , Architekt, Redogg, [B.Z.H] Psykos, Saoas Alcapone, dud