Did you create
TXT records on your domain for your DKIM public key? If not, there's no way for Gmail to validate it.
Do you have any
SPF records setup to validate that your server can send emails from the domain? Is it configured correctly?
There are also tools on the internet that will help you troubleshoot issues. A search like "DKIM validator" should give you some tools to verify the DKIM record is visible.
Also, what do the headers say? Sometimes email servers add headers to indicate the reason why a message was sent to spam.