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, 4am.joshsummer, Afrodight, [B.A]BLITZKRIEG, thiago_eitner, deniszagan, lukabeatter, Mr. Dad Guy, Braindawgz, kjs921121, gökemakpence, Squeeze, Rokstar2002, Professor Membrane, tizubogdan, ariano113, ariano113, MithomanoWS, ♫ Wub Master J ♫, ChalkSauce, bbenjamin1702, [F2P] Rajderr, TrBarney, Sir Cornwallis of Cobbsburg, djokersted, jorgenieblas, jorgenieblas, ★Pepper, ★Pepper, mamma.knullan123, Vtgordon, hate, Czerwony, Spazter, DarkShrek 52™, The Unknown Ninja, Ashot, anal4pay, z.huber, Povinnostak, micah, Captain PogChamp, GAGARIN, ~~Snorgan~~, Lt. Soap, whenbananasattack, Cipher_zero, Dr. Ermac Freeman, Dr. Ermac Freeman, Gerry S +71 others