This download looks like a map. You need to place it in your maps folder inside the Garry's Mod 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.
DenCHarA, PC_MASTER_RACE, LLaci3, andyrainody, Gorelov27, awert097, jaysonmarcon, Remulux, Fabulous, Rick, Sr.Underground, _M4rckox_, coghlanjoe2, DarkPikachu221, DuendeCarecaCiganu, tmdgus7011, chaconrules, jacksonsp117, Iho12345, Kingsbury, =(FLM)=Marsul1990, [AP] Satoz, [KR]arc, Dawson, wsps70002, Microsoft Warrior, Yoshino (R3B0RN), Han Solo dies killed by his son, Han Solo dies killed by his son, Radke, TheAdrianbbc, DjCevuiMix, EpicLizzard74, FliesWithWind, 1934366311, maty-reyes, ShadowGhostTR, f.letelier.x, TheTrololoshka# :3, ky_ky_epta, DazenN, Kapral Levi, ~[D]oc, Kimdoldol2, Colourful Anal, 지나가던 잉간(???), andywong789, Zane, Gabriel, Freiheit +58 others