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.
Foxter, Thomas Patoche., Jeremylefou, ralphheemken, Cheumklienméclenichien, ✰Not-6248✰【TH】, ^1Alexis ^2Pro, Rager.exe, [Ectasia] Sibau Angelo, Moha, † ℒ, TaBoomer, Thomas // Reezo, Hikk/a/.exe /b/ /h/, Warpath-the-drone swe, [ZworldRanger] OPX, Sidroz, TheDarkJudge