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G4P GUI Builder v4.0 is compatible with Processing 3 and G4P v4.0
It will be a few days before it can be installed through the PDE but if you are in a rush you can download the update from Sourceforge
If you do that then unzip the file and put it into the tools sub folder of your Processing sketch folder something like this.
Notice that my PS3 sketch folder is called Processing 3
Enjoy.
Comments
It is now available to install via the PDE Tools > Add Tool option