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.
DeadPool69, moca5493, zanofa88, tee-jay, Empire04, hawkboy9000, Vanman, TheRealWane, Rick and Morty, UNSC Dawnforge 2142, VETERAN, in-flix, nanashi86, undefinedscott, Michu351, GeneralGraybeard, Savagely Random, Savagely Random, alkalia.alkalia, KARA\0 ♡, Saido berahino, rstclm, keep_good, Bulanda, The king of cinders, MrKapral, Elvis, ♫ Wub Master J ♫, romero1718, Kaganation, Festive ☾Moonman221998☽, yan.zotoff, ChalkSauce, falseprophet141, falseprophet141, falseprophet141, Wolf, Spunky Mei, roman_mm9, 25082000ilia, Ultrapolygon, Vault Boy [RU], 1246368102, TheMadKerbal, Piro Winfield, pstratto, pstratto, Hyperaxe, justincreator, justincreator +38 others