This download looks like a map. You need to place it in your maps 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.
cristinags71, ThePrimeOfPUPITMASTER, demomanyt YEET :p, Randon Fandoms, Monika, onlineleon, mfaudziladhim599, leon, xoraz, Harambe Uchiha, RAINBOW CRASH, garga, ~-~-- LittlePip ~~--, anpan3chibi7, Romaschkin, RainbowGameDash, FTS57, roekiyah18, thesparemare, mangl.valya, mossesx, King_CreepaLot, sosipisoskuimacev, arianasheghi2377, michaelsandoval227, kirchhofflukas0, Marci, pp08250, miloszko, Boterham, Fershy, Sweet Corruption, NightMena, mr.lenrd, Record, Evil Mortica, alan_szwagier, EmmaCrafter, носкиллз, heiko.klopfer, danilstarbird, qwertyuiopkl5567, oswald2001HD, maciekrotman1, maciekrotman1, re15revisited, frankdavidbg3487, elektrika7000, Tyranicus, alvaresl.matheus +78 others