This download looks like a save. You need to place it in your saves 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.
What The Fuck, abelfer2006, Mike Schmidt Fnaf1, miguel300477, burnellbraydon2001, Q0GAL-7QJLH-DI08I, Q0GAL-7QJLH-DI08I, Connor, kangur845, sss, sss, Phobos, Kermit The Froggo, Pug Dude Studios, Saysay, swagNswift, risttomarcus.urb, stevansremac04, ariano113, Hectorencio12, Determination, marinmarcano2002, He||oween, joawnai, DeadRainBowDash9124