Forward mail and obey SPF and DKIM

[IL Ka]

Hello.
I have postfix running on linux box.

I setup OpenDKIM with both smtpd and non_smtp milters.
I also set my address in DNS as permitted IP for SPF.

So far, so good.

But I want all my mail to be forwarded to gmail.

Some user sends me email from user@some_sender_domain.

If I use .forward or alias, then postfix doesn't change "From" header,
so gmail believes email was sent from @some_sender_domain.
This domain doesn't have my box IP as permitted in DNS, so SPF failed.

I can change header using headers_check. But then DKIM signature
would be broken because some_sender_domain signed email and I changed it.

It seems that I need to:
* Change headers
* Sign email with my DKIM
* Forward it to gmail

But milters are not applied on forwarded emails because they aren't locally generated (or I failed to configure it correctly?)

I can fix it using custom script that reads my local email
and sends it to gmail.

But how can I do that with postfix?

Ilya.