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.
christiansalom97, zanofa88, rosantosguelfi, alexshulski, alexshulski, Danjochek, ☢️ DeltaOne ☢️ csgolive., re15revisited, rmn1981, cefergh, patryk9903, ดิ่งครับ[TH], ANDRIANtheGAmerZ, thirsha4242, hjmen32