When GmailAssistant launches on Linux (e.g. Ubuntu), it fails because of an unexpected error --- java.lang.UnsupportedOperationException --- and the exception stack trace begins with java.awt.TrayIcon.
The error occurs because the JRE is unable to support tray icons for the desktop or window manager that is running on system.
Make sure that you are running the latest JRE from Sun Microsystems for your system. Tray icons are reported to work in Ubuntu + Compiz with JRE 1.6.0_10-beta.
Ensure that the tray area is visible on the desktop.
Turn off advanced desktop effects (e.g. Compiz).
Java SE Downloads
"Tray icon support in Java broken when using compiz" on Ubuntu Forums
"XToolkit.isTraySupported() result has nothing to do with the system tray" on Java Bug Database