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.
ps720936, pablosv2003, 塔尔斯, IbroLI, Digital_Ka0S, r.julio115, diegoricardo1915, yazansbouh, kadirvolkik41324, evilgood2010, Alpha_ZiDaR, jpstirad, n0ttii, Tyler, huyken93, TheCatHL, william526, Kary, firmamentum, savage, tau2mcc, ♫ Bratishka ♫, ♫ Bratishka ♫, Skunky, Rycerze, gamemaster112ful, SCP-035, defiatron13391, StevenR380680(Zulf the Wolf)