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.
Skefil, titoshafy2006, Salvadorc17, systematronics, LIFEofDOMOVOI, japanrailway90, greenlender, BigUncleCherry, loginob_nikita., re15revisited, re15revisited, Andrew, Brad515, jimmyfoo50, sebasg12321, kyshon25, mohammad84khani, tlbandicoot, Sir.SoLDIeRs | tftrade.net, Ryn Wolfeh, renandantas71, loic.c2144, G-MON, Serial Killer, victor-mendes-tuba, greenhunter45, mikecoros05, UncleSHREDDER, truskiek1, not VorteX, Warpath-the-drone swe, NikoIsCool1, Patrick Grady, Designated Kitty 03-052