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.
Mario MANN, Zatsugi, *g0md0r1, YetiBomber101, YetiBomber101, daniel-dan201, Anzhi_shuijing, Honey-Badger, shizozavr, =wHaT tHe=, 27lueddi87, lifejano, dimon12651, dimon12651, koroglualperen014, koroglualperen014, tigran.abazyan2001, THEflicus | gamekit.com, [ ], [ ], Rodos, BBBPL, mtart97, jlowe3, Lucho, Lucho, Lucho, Lucho, Lucho, Tyrial, kidjaeden, Chung, (2018) edenarmar907, re15revisited, weirdo1921, TheCrazyDoom, armanzhu96, Braian_99, Derelict, Derelict, Elongate Card308 Official, UltraBlaze360, LembuGhini, maks.maksik2013, That guy named Mint, scar.rab61, RiGBY, Infected with Chang, Wallnut, xiongliangtian123 +25 others