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.
gamehad., lil kid, ⦕•̶๖̶̶̶ζ͜͡TroPicz, marines, SGH-ADYOIY, rainbow_frog8, Water Melon Cat, 三玖天下第一, obvixus, manuhopaba, Oofmancool, MONSTER JAMMER, CinnaBon86, trollman111, Leoperden, Joey Grundle, Hawaii, roblox gamer255348, TheDoofusPookis, pinocc123, thatothergamer, Fruitcake, Kovxrri, seb6416, gum, 46e, Airwaves, ﴾ Shikatsu ﴿, IntoxTurt, May, xqq, igla, EF7, Loki, LawrYtY, Aidan, Foxy, Bean Box, MrR4P70R, Catt, NOMAKAR, Lil Trans, , bradtumber1, Endvad, _remixx, mosque cleaner, enjin, Nothin' Special, FakeFlyer +1971 others