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.
Akarin~!, Sonichu, zhizhang12, Leoplace71 Gaming, armani, %%%, ♥ Kitten UwU ♥, Fattata, legoconsoleboy [GameTame.***], weathers, raydscott, Wontairr, Gregorious, dward99, bmrmar2003, SmkNade, team footrest, Brooks ☦, cheese, Sekuza, Froodoo Foozboo, KrzysiekSiemv, frauber3, A.j Mason, Bear, Notrams, kity, Stidk, Groove [FranceRp], Franco M, Moku, General Toast, Flynn, I pro tomosky, Darwfer, < blank >, Bumpy, jvyden, Schnitzelfuchs, vic_gx, deef, deef, deef, deef, deef, deef, coyote, Svennarthan, NeoLin, Robokid +8 others