The defined means for expanding and collapsing tree nodes in GNOME/GTK+ is to use the + and - keys ("plus" and "minus"). In Swing JLF, the defined way is left and right arrow.
When JTree nodes are displayed in the GTK+ theme, they should use + and -. I suggest that they continue to also use left and right arrow, so that users familiar with the JLF will find that mechanism continues to work with the GTK+ theme.
See the Sun GNOME Accessibility guide at http://gnome.ireland/doc_proj_team/reviews.html#a11y The section on "Navigating Collapsible Lists" in Chapter 2 shows the GNOME/GTK+ key sequences. Note also that Backspace should select the parent item.