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.
844box, Bedrock™_HEAD, re15revisited, re15revisited, Heavy weapons guy, Z_A_R14, ariano113, ryslanchikkk, rmn1981, raymond1922, theAwesomeSponge, KyuuGryphon, decadent_uk100, 2R1Zt, bonnieboy11, Sabun, Gianvix, Gianvix, ddb, Albino-Sick-Stick, l.maire c, Kylharc The Sinistar, MonarchMonk, Naldo, GENERAL FAILURE, mishaizgrezino, Badhamknibbs, szikora.david, フラックス