Try using Email Sender (https://xeams.com/email-sender.htm) to send messages to gmail.com. Put gmail-smtp-in.l.google.com in the host name for the SMTP server. See what step does the connection fails.

If it fails right away, or before the MAIL FROM command, the problem is NOT related to your DKIM.

If it fails right after MAIL FROM, it could be related to SPF

If it fails after DATA, Google is parsing your email and is deciding to reject. There is very little you can do on your end besides sending an email to Google's support and ask them why they don't like your email.