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.
DqrkGFX, Gundog Giantsbane of Gilead, Silverplayer, Bender, KaltesSnickers, IKERNEII, day1, Teilzeitrekktor, [FL:RP] TASSIA710, Jake, XxMrYooxX, Malyki, Acel, DerAnonyme, Capt. Freibierfotzn, ForEverSelim, Pr3xx0r [GER], MOLDY OLD GREG, Classy^^, Mickeey, gidoguy, Mario, Sirzento, cygiel, |SG|Chris-Skulls-Gaming, xbeastguyx