Gajim don't use StartTLS.
When I connect with server, Gajim don't establish TLS connection. I think that it, beacuse is stream:features on the first place is <auth/> and <starttls/> is at the second:
<auth xmlns="http://jabber.org/features/iq-auth"/> <starttls xmlns="urn:ietf:params:xml:ns:xmpp-tls"/>