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.
[hoovy] supertravman11, GrandJury trade.tf, Mister Crowbar, activatorulvoltzengan, Dott2000, duduklusener, Rosalina Satori (FatalitySonic2), I'm not a duck, I'M A SPY! [AYY], ☭ Lord TL ☭, JuanmaGamez324