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.
NoName, Fedelefante BETS.gg, Profile, Cokacola, [VDDS] K.N.cfg @ Jeahy, 밀크|milk, the great papyrus, Sgt. B. Blackburn, SnowSmasher csgobig.com, ∂σɢє, Robin, Sloth, Useless, DaZe Fungus, kev6423, ???, [☭] Alo0iis, ❤Senpai❤ Pika, syno04, wolfboydaalpha, Mr. Snow, Supah Maryoh Brudder, gtakiller33, nickles.101, 老貓 - TheOldCat, 老貓 - TheOldCat, ThaCerealKillah, Alukard, 살선생, [FR]Sharky, [FR]Sharky, ✮✮Black Gold Saw✮✮, Bilbosandwiches, dowg, dowg, Broken_bones658, .onion, www.andrews, sahoka, Binjuu, Magmabone, [Nazi Spooder][BMC Guard] WOW, Darth_Starlord, ;;, 제라드, 59, Raze., Senio Ally, j0257809, Ms. Ṅıġḣṭṡhåđe +140 others