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.
TVSIS1WITHALL, (_BLYATMAN_), Rattman2004, ArticBajablast33, 392189, 392189, t.tenczer, commandcmd, [MG] Masterkill, Xacky22, Bush did 7/11, Pepe, SongbirD, engineer gaming, ninja.sd, kefir4uk, ., blerimiseni, Grand Dad, gmod lover ( ͡° ͜ʖ ͡° ), Ano1010 Slenderman, bynoment, manaos-man, beeshwhopper2, Hickya, Hickya, xboxgamer1579, Malon, Daywalker, redboy4313, redboy4313, asia29r, Glf20, Kamper Van Helsing #Biden2020, joshuafidelman52, Col.Sam, hickya32, Obey, Secret Lab, kylo ren, Tinotin, mr-cuddles, mr-cuddles, YoyoGamer998, ericeric06, ericeric06, sentryturret445, anhhung567881, Jack Baker, bensonphilipps +14 others