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.
PiggyPlaysz, HO CHI MINH, Ace of Kings™, _[TSK]_RustyMonday, Yttik, ПЗРК, maksim-myh1 [Angry Pilot], Potato, Potato, Smoky^-3*, SpecialOperations44, Ultimatesda, ◘↓♣♣♣Abdela54♣♣♣, Vokster117, IcyDragon617, damontbridge13, Vortigaunt