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.
dunarintualex97, ItsDarkwolf64Real, El Ingeniero Loquendero :D, Aizawa, kefere8725, Socalenergy, edgarsgrivins, raul_balverdi19, Fernando280, insert quality name here, tengounaplantamovediza, commander shepard, ApprenticeofLink, daniilgaming_yt2acc, djlarryt, fariasgomezbruno, OLIVIER Key-Drop.pl, DSBrony59, Demirmann, badaidp, SelkTheOne, Wiltz, nexgamer303, chochin7675, manzoej, arnyvasquezperez, FishLiver2143, Commander Con, Commander Con, Commander Con, Spookmeem, zshaun97, GASSIOUS DREED, reconditewarmowl71, ilman.skoba, (~AYE~) SimonLuck, Bernard, muhammadalifafiq123, muhammadalifafiq123, qxabd60, Ze_Mental_Gentlemann, mauro.fernando.gonzalez, vizeritblue, , Nekyto, Nekyto, rumoragui, Lucazo117, ВАСАЙ, ZKA +621 others