Several efforts are out there attempting to marry Java and Gnome which is quite an interesting topic.
SWT is a brand new implementation of a Java GUI that uses native bindings to draw it's widgets. It's available on most platforms including Linux (GTK2 and Motif), Windows, MacOS, to name a few. It's also the GUI behind Eclipse, the Java IDE.
Java-GNOME is another effort to allow Gnome applications to be written in Java. It supports both GTK and GNOME bindings.
On top of this with the latest release of JDK 1.4.2, there's GTK Look and Feel support.
Will be interesting to see where it all goes :)