In 4.3.1 (latest version) the user can specify the global font face, style and size for display (buttons, labels etc.), input (textfield, textarea and password fields) and slider (values displayed in slider controls)
The methods can be found in the G4P class and have the patteren
G4P.set?????Font(String face, int style, int size)
If using GUI Builder 4.3 these methods must be called in setup before the call to createGUI to have effect.
In GUI Builder 4.4 (to be released) these will be changeable in the design window. NOTE GUI Builder 4.4 will not be able to load and edit GUIs created with 3.3