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.
gio23456, minhchxhcn, sans125, emreari7ea, GENTLEMEN SPY[ASİL TR], azizgaming5, SYKESCZ, (((EZE)))(((TIGRE))), molekula12358, Dr. Door, Flying fighter, klark-azek, T o T, ryanpeterson933, Arexa, bbbazsi22, sawickakasia1, Anonymous Player, eduardoviasvaldez, joepooh69, EginKaman