By default all messages are queued in Gabber's spool. This means that the message will not automatically show on screen but will instead wait to be opened. If Gabber is closed before messages are viewed, it will store them until the next time Gabber is run. When a new message is received from a user on the roster, the user's name will flash with an alternate color and the status icon will change to a flashing message. To access the message, just double click the user. If the group the user is in is not expanded, the group name will flash. In addition to roster visual notifications, the GNOME status docklet icon will indicate a new message. Double click the icon and the new message will be viewable.
Customization of queueing behavior is available from the Messages Preferences.