The addons folder is no longer used like it was in Gmod 12, if you have the Legacy Addons addon from the workshop, you can still use the addons folder with models, but you need to rename the info.txt to addon.txt for it to work, but that's about it sadly
For maps that are in the addons folder, just take whatever is in that map's folder and put it in the garrysmod>garrysmod directory, you can also do this with models, but with the Legacy Addons addon, it isn't entirely necessary
In most if not all cases, any addons that use LUA or something most likely will not work even if you take them from their addon folder and into the garrysmod>garrysmod directory due to the changed LUA coding, so your best hope is that it's on the workshop or is going to be on the workshop
BY THE WAYIf you ever want to extract files from the Workshop's .gma addon file format for some reason like wanting to use a model in the Source Filmmaker or just want to listen to sounds or music the addon has, you can use this handy dandy Gmad converter
https://dl.dropbox.com/u/27825152/GmadconvGUI.zipThis is a video that shows you show to use it to extract the files and use them in the SFM
http://www.youtube.com/watch?v=v94aHWs3S0I&feature=g-u-u
