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.
Albert_WeskerT, nate the mlg pro, SMelen, ❬«G§A» JackSnakespire, Ronnie Paints, dogukan_aksoy222, re15revisited, JRed, gobing2003, misaelbravo19, tonywan2000, zombrex70, kyshon25, phuocthinh the super vodka vn, OneBaddy, j143803, hewlet, charlie_lee_rhee, ♤Isaac Clarke, [ISA] raikiri 康纳, nongteam_seka, nongteam_seka