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.
re15revisited, 등짝을보자구나, XxDrFluffxX, Krystian02sl, fofo633, Patryk_YouTuberPL, RageVolume, rolcsi08, durmusertu4, jalalheidari83, [FP]ICEMAN, fernandesmanuelc, IamBrett, sukoisu47, sukoisu47, Oslo, Nata108, WickedRabbit, ada1453, martin.kosecky5, falcon blade master, rayan919, Vince, [DIR] Edward Richtofen, FiLzO, jimdandy.roofcandy, Dr_Triman