When you say "move M-x customize to gtk widgets", which text based would you replace with GTK widgets? Customize certainly doesn't initially seem as friendly as a traditional preferences dialog, but I would be hesitant to lose the "it's all text" navigability and consistency.
Having a friendlier, good looking "Preferences" based on gtk would help newcomers. We could keep customize too but a new panel with simple familiar look would help a lot of people to get into emacs.
