I shouldn't have to say this, but this is an 8+ year old Adv Dupe 1 file. Do not expect it to work.
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.
Spherical The Rainbow Pegasus, luismoragomezilovemymail, (THE WOLVES)CYKA BLAYAT, nathan.elloway, , drinking for a good harvest, GillKnight, mrstachycowgaming, TrueHero, Lil Admin, ericsage124, Wunker☿, Nekyto, wolverine041269, WARRIOR PRINCESS GUCCI, ERROR, ERROR, A Random Person, brody_1991, toloer2345, EC BUCKSDROP.eu CASEDROP.eu, MDPlayy, Sicarus, fidzPKa, jkyyooo, † BL † St0mKa, prosto Nazi catϟϟ, Gaster.ink, bARRY THE BANDIT, xZombieTheGamer, Holy Mackerel, carlosgold345, GON, Tombstone_main, Tombstone_main, Flared, 2nd Lt. Ben, < blank >, chriss2300, SukaBlyatHARDBASS, SukaBlyatHARDBASS, 갈록Galok, BeepBeepBoop, צפרדע, treyshawn, Стасян, efeh4n3r0l663, sedatsedatasdf, Saitama, Success kid (pavelake) +246 others