The GUI editor is deprecated so I would recommended avoiding it.
For examples see the tutorials and check out the examples directory in the top level of the repository. Most notably the DemoRunner has almost 30 different GUI demos.
Another useful resource if you’ve just got started with JUCE GUIs is this talk from the original author of JUCE. It’s almost 10 years old now so some things have changed but not too drastically I don’t think. It’s great for introducing some important topics and useful ways to work with the JUCE API. If you find there’s anything you don’t understand from it there’s an excellent community on this forum willing to help.
If you really want to build your UI visually then as mentioned above Plugin GUI magic may be worth checking out. I’ve not used it myself but I know others have had success and the creator @daniel is often found on the forums answering questions.
If you’re planning on building a DAW, for all the non-GUI stuff it’s worth checking out Tracktion Engine.
Hope that helps.