I found that - when sending a spamgourmet email originating from one of my gmails to another gmail account of mine - I could uncover my originating (supposedly hidden) gmail address:
Received: from mail-vc0-f171.google.com ([209.85.220.171])
by gourmet7.spamgourmet.com with esmtps (TLS1.0:RSA_ARCFOUR_SHA1:128)
(Exim 4.80)
(envelope-from <myrealaddress89@gmail.com>)
In the recipient gmail account, I click "show original" and my "hidden" address is there, even though it was sent using spamgourmet. I don't think this has always been the case, as I've tested it occasionally and I believe origin info was hidden in the past. Now.. not so much.