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.
Kbredeson [TC], Reck, [CNU] [S37K] jacobsf, re15revisited, Zxvnm4, anhducemhung, Nib, Faliman, Faliman, Trainjumper❤, [STRIKE] Sabo (ﾉ◕ヮ◕)ﾉ, Tryingdieing, Lλmbdλ, ✞ ΞIMGΞ Kira.org™ ✞, buder28, Vergil DmC, Drumdevil, Vox Maro, Hoffa