Since Ubuntu 13.04 suppress system tray support you can use this plugin to place gajim notification icon to indicator applet.
This plugin is transitional and must be replaced on MessagingMenu support when gajim migrate to GTK3
You must install python appindicator binding library (python-appindicator package in Ubuntu) before enable this plugin.
Plugin use icons named:
That icons must present in to your system.
Use special plugin, that manages automatic download and installation of others plugins, it is called Plugin Installer.
Just activate 'Appindicator support' in Plugin Manager and you must see gajim indicator icon.
Existing bugs and feature requests are [query:?status=accepted&status=assigned&status=new&status=reopened&component=AppindicatorSupportPlugin here].
If you have any issues, create a [/newticket?component=AppindicatorSupportPlugin&owner=BDenis new ticket].