This plugin bridges the GNOME Presence settings with Gajim. If you set your presence from the GNOME panel Gajim will synchronize its presence with the new settings. If you set your presence with Gajim the GNOME panel will be updated to reflect this.
Extract it to ~/.local/share/gajim/plugins/ (create the directory if it does not exist).
mkdir -p ~/.local/share/gajim/plugins/
tar zxf gnome_session_manager-0.1.1.tgz ~/.local/share/gajim/plugins/
Start Gajim and check the checkbox GNOME SessionManager in Edit → Plugins.
Or you can use special plugin, that manages automatic download and installation of others plugins, it is called Plugin Installer.
Existing bugs and feature requests are [query:?status=accepted&status=assigned&status=new&status=reopened&component=GnomeSessionManagerPlugin here].
If you have any issues, create a [/newticket?component=GnomeSessionManagerPlugin&owner=philn new ticket].