To avoid confusion with the G4P library this tool is called G4P GUI Builder or more simply GUI Builder. I will use the latter.
GUI Builder is only used during sketch creation so is completely ignored when the sketch is executed. GUI Builder also creates a second tab called gui.pde which is exported with the application 
. It also creates a folder called `GUI_BUILDER_DATA which is also exported but will not be accessed by the app.
Now I have an iMac desktop and W10 laptop so I can’t reproduce your tests but I created a Processing sketch using GUI Builder that had a G4P button and G4P label. On each machine I was able to export the sketch and run the application. Processing on Windows does not support exporting for mac but I did create a Windows export on my iMac but it failed due to a JNI error.
Sorry this doesn’t help much but it seems to me that the problem is not related to GUI Builder or G4P.