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Which GUI is the simplest to learn and implement for GUI interfaces?
It depends on what type of application you want to make e.g. desktop, android web
If you are interested in creating a desktop application then I suggest you try G4P because it has a companion tool GUI Builder which allows you to create G4P GUIs in a graphical interface. The website also has a number of videos showing the builder tool in action.
Obviously I am biased ;) but the alternative I would consider is ControlP5 which does desktop and Android.
Both G4P and ControlP5 have been around a long time and have a significant user base which is useful if you want forum support.
Thanks, I'll try G4P first.