I know how to whitelist most other applications, but adding 'Gajim' and 'gajim' doesn't seem to work.
Does it have an appindicator?
Well, in a strange twist, killing Gajim and restarting it seems to have worked.
Apparently the correct value is Gajim
.
Maybe it'd be a better idea to install "Ubuntu Ayanta Integration" or a similar plugin instead. See http://trac-plugins.gajim.org/wiki/UbuntuIntegrationPlugin for instance.