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.
Doctor Dakka, osos95, Mr_AL, komarov, QTG坑, zolstarym, OptimusPrime, Avalanche, G̶o̶i̶.̶, Patrick Jane, Appamongkhon, Viktor, Xxx_DeathHammer_xxX, Xxx_DeathHammer_xxX, [☣PoNy☣]HardcorePartyCz, Raven, Keshwoof, Nulus