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.
Baron FlameRP, jonardkyle777, Myrtle, stolid bets.gg, Storm, [GL] Train Overlord, derekSTUFF07, despinic.s, [NWA]- Jake, bautista13angelica, krystian.kosim, monna_george2007, DeadRainBowDash9124, Seph