This download looks like an addon. You need to extract the downloaded zip and put the addon folder in your Garry's Mod addons 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.
Ashir PL, 999Pawel999, Koopashell TMK #Meem, NaziBonzi, Doctor Mobius, Doctor Hunt #MAGA, Jurisdiction, Jurisdiction, R1cardo, l.maire c, TF2-Reddpie, Warpath-the-drone swe, theundergamer, kairukun93, kairukun93, kairukun93, Dis Dolphin Guy.., Dis Dolphin Guy.., The Unlucky Medic, NotWaitMe675, PeculiarPsychopath