Not in Spam.
I followed the guide to install
Postfix Gateway on Debian 6 some months ago, and it worked normally until few days ago.
I guess I haven't setup any SPF record nor DKIM, but I think that if Google can't verify the email it would be on the Spam and not just disappeared, so it should be another issue.
I just tried the mail forwarding from a Hotmail account and it reached the GMail inbox without problems. I think that Gmail discards repeated emails or something like that, and now I cannot send a test email from the same account that will receive it.
I will check what happens with the incoming emails. Thanks