Do you host email for multiple domains, or are you an ISP?
If so, here is what Gmail says you should do in order to forward to them:
If others use your service to send mail (Internet Service Providers, e.g.), you're responsible for monitoring your users and/or clients' behaviors in the following ways: You must have an email address available for users and/or clients to report abuse (firstname.lastname@example.org, e.g.). You must maintain up-to-date contact information in your WHOIS record, and on abuse.net. You must terminate, in a timely fashion, all users and/or clients who use your service to send spam. Please note that if you do not monitor the sending practices of those using your services, your delivery reputation may be damaged. We are unable to whitelist IP addresses at this time.
Even if you follow all the steps above, you can be blocked from sending to the Gmail servers, because...
Your users will still not understand that they are forwarding from their account on another server and
mark the forwarded email as spam in Gmail.
Smart spam filters would probably grock that the email is coming from an address that is forwarding from another ISP.
It is not like the ISP is changing anything so quickly that there would not be a pattern to match on!
You would think a company that lives and dies based on patterns would grock that :-)
I hold out hope that future programmers and / or email administrators could figure out this intractable riddle.