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.
kunanonoradee, Q0GAL-7QJLH-DI08I, dmaksar, Albert_WeskerT, EľKazel, sebizalog, ︻芫═──, bobacrome, Getrox, jafet.nimrod, gmod_ww2, yasser.05yakh, Marius!, valentin.camusat8, yollya, Heckelex, Retr0, pecaros01sp, Xxx_MLG=SUPER SCOUTH_xxX