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.
⠀⠀ ⠀, Camsvdb, Goharjavaid8, lizbon1024, kemleyo-salemsiz, Devidion, Koopashell TMK, ❖ MAX ❖ I ❤Toriel, senka02, The Shpee, Crociax, Red Robot, fakrul484, golden freddy/fredbear, ReaperX, [MLG]StoopedFoxUA, Wiremodwizard, onzowear700, gordeev.alexey, Cute_Bonnie1987, ToastyCritz trade.tf, CTX, Princess Archimedes(Manly), Meta Tomato, BigWAT, BigWAT, Mozzila FireFox, some unfunny skeleton, manman, Warpath-the-drone swe, 지옥문을 여는 방법., =PP= 419, Kairamen, The Era of Plem Plem, Borisov, ales.tomse, bradpl1, Skeleton[PL], TheTimothyTruong2002 Official, President Donald Trump