This download looks like a save. You need to place it in your saves 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.
gtfhrhg, metrald1, deathrider178, Kittiphanleo, hgdgffggfgd, Ice, jocagmer, Luge May Kamote, deadLC, falseprophet141, rmn1981, fazafreeman, sunny176, bevgre72, nasr1355, ericeric06, savinseby44, xtreme-g, Reed Timmer, Juegacubo, manugamerpro33, ins0mnius, guesplane37, ghibusi_david, justin.simon.cernowski, mohammadstar202, emrezobi, diamondmaster055, Pizzacat17