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.
ashleigh-mcstay, CarCrashCompliation, grayddon, edwardsroxanne509, Recxczyy, Testsubject00001, djuka2008, lissandrocastillo6, byun0223, Silen, rexzy, camijc1104, juliarni2019, Icespiders, domuinor2, lonesometown, thai_jetse, mohmohazdz, desmetbryan8, SomthingStupid247, krut.daniel, tuliprose, Dhzolv2304, ackermannjackson49, RocKsTaR, slacker021, demoencoderecords, brawlleinst466, me9635728, PutoReaper, Mario Mario, niwp13024, DooleyYT, amaestrar4721, ZenithCheese, dham3830, davidstroescu040, ben27lobo, thanadit08725, andrydidier39, barlu.micheal, Stre4k, FAZEJONASGAMING, monochorme_cube, koatgamer2010, darkmeshadow072, GihouDrogado doidao das parada, debbierobinson421, Pummak, kalasatama6629 +21720 others