Re-upload from garrysmod.org - author unknown. Please report if this was your upload and want crediting.
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:
tracks <-- Addon folder!
Mermit, Szymon, JAN.HYMR, Бандикут Крэш, GSM, Masskeeler, a stupid ugly doo doo head, a stupid ugly doo doo head, Papkee, Hank Kits, Hank Kits, Hank Kits, Hank Kits, Hank Kits, Hank Kits, Hank Kits, Hank Kits, Peavy (<3 Lizzy), bjornbouwmans97