Bug 6477839 identifies some issues with JToolBar under GTK L&F and the Nimbus theme,
but there are other more general issues that affect all themes:
  1) Native GTK toolbar buttons are square by default and have a certain amount
     internal padding; currently, Swing's JButtons retain their rectangular shape
     when added to a JToolBar, and their padding is inconsistent with native.
  2) The SynthToolBarLayoutManager that is used by GTK L&F does not properly account
     for insets, which means that toolbar buttons currently render on top of the
     toolbar border, which looks really ugly.
  3) JToggleButtons that are added to a JToolBar have their border/content painted
     when in the non-selected state. This is different from regular JButtons and
     native GTK toggle buttons.
  4) The JToolBar itself does not seem to be painted properly; specifically, there
     is an extra border around the toolbar portion that is not present in native
     GtkToolbars/GtkHandleboxes.
One more issue to add to the list:
  5) Native GtkButtons do not paint their border when in a disabled state in
     in a toolbar setting; disabled JButtons added to a JToolBar do paint their
     border.