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.
Zac ATK, $DORF$, JarynGames, JarynGames, Xerox5739, AfikPeretz, rmn1981, dmsaroka11, <3 Bieber <3, PhoenixMedia, PhoenixMedia, Alpha Tactics, jasparLP, BOT Ron, BOT Ron, ☢(T!my)☢, jeniffer_jsr, adabass, MR.Jackie, reaktor24, MrChillaHD, BIOMAN, Riszzky (EGRP), Riszzky (EGRP)