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.
George, 19914602909, elepepepepep, NinjaGamingDev, checcoprobello90, Korasosbar, ⍰⍰⍰, Bieja1, hristya.vaksna, t3g0_typv:), Smexycheese, wooden log, Quadrazid, MichaelTheKing, SRslendy, < blank >, TypicalName, PowOwu, Buh, Эстетика Грусти, jellyorange647, Toxic, DallerenIn4K, ShadowKing, Erax, cjw3751, Deku | IFN.gg, freddie mercury, damianwieczorek, damianwieczorek, simfus trollface, b.fiala12, The Mighty Edwini, Slime the Protogen, EderRadien, ZeedZadZodZulu, Tithome, HugeBandit1010 (MOAT.GG/VELK.CA), [VE] Hors5G, CiachoGames, skrr, Luck6, ĐĘPŘĒŞŠĚĐ, Galahad, soxfan04, kak.oleg, DumpZoomer, Sharkboy439, FBI _Parody_, tomsmcdonalds88 +215 others