When GMOD 13 was released, Garry made the switch to Steam Workshop instead of his garrysmod.org site.
Though the Workshop also requires you to login to download, which is a flaw. Free content should be free.
Therefore I have provided a method down here which allows you to download the addon files without an account!
Please enter the Steam Workshop link to an addon you want to download,
or just the pure workshop ID of the addon.
Download this little tool to easily handle the files you download from the workshop!
It tells you the type of the file so you can move it to the right place.
GMS goes into GMOD\garrysmod\saves
DUPE goes into GMOD\garrysmod\dupes
Please note that it is BETA