No longer maintained
It probably still works, but I recommend using gmpublisher.
Garry's Mod Addon Tool | GMAT
A light-weight Electron GUI for gmad and gmpublish that allows you to create/update/extract Garry's Mod addons very easily.
As of v2.0, you can upload and update your addons, as well as extract GMA files.
Instructions
Download a release and run gmod-addon-tool.exe.
Troubleshooting
Make sure you follow the wiki's directions to the letter.
Common mistakes you should be aware of,
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your addon icon not being a baseline JPG will cause your upload to fail, which can usually be fixed by exporting your icon in Paint (Windows), Gimp, or Photoshop.
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having an unallowed file (or an incorrect folder structure) in your addon's directory will cause your upload to fail, here's a list of allowed files which should also give you an idea of where to place things
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not being logged into Steam will obviously cause uploads to fail
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you must have Garry's Mod installed to use this tool, since it utilizes
gmad.exeandgmpublish.exewhich can be found in/GarrysMod/bin/
Linux specific
If the "Update existing addon" button is stuck in an infinite load, it's possible the libraries required for gmad_linux and gmpublish_linux are not correctly linked. This can be fixed in several ways and usually varies by distro so I cannot provide direct troubleshooting for this issue. Google DuckDuckGo is your friend.
Otherwise, create an issue or send me a tweet/DM.