The issue with workshop freezing may be a need to manually install the file of the map we are on. I had the same issue. Once that's in, it's a matter of patience. Typically for me I time out the first round I try to log in, get in the second round after like 8 minutes.
The Gmod update screwed up some stuff.
New subscriptions no longer download to your addons folder. Your map's .gma file is actually in a new location, for one. The location of your downloads is now in some random ass folder contained within your directory here:
C:\Program Files (x86)\Steam\steamapps\workshop\content\4000
If you sort by date modified, each of those gibberish folders will be in the sequence which you downloaded those addons or files from most recent to least. If you take a look at the attached Annotation 141820 doc, you should see something like that. If I click on the most recent folder I have there, because I have not downloaded any new addons since I downloaded the map, that is where the Zaton Remaster .gma is now stored (See Annotation 141949). Try deleting that (fyi make sure you're unsubbed from the addon).
Do not resub to the addon. Go to your Gmod, then Multiplayer, then Legacy Browser. Open the console there. Run the command "menu_cleanupgmas". Restart Gmod.
Resub and redownload. Restart Gmod again. Try connecting.
When I did that, I failed to connect the first time (timed out) but succeeded the second time after waiting much longer than usual.
Hope that helped, and please give all informative ratings to
skylion
who gave me the guidance on this initially