GWIA - configuring to use a postfix server to send outbound

So I followed the instructions in this link 

www.digitalocean.com/community/tutorials/how-to-install-and-configure-postfix-as-a-send-only-smtp-server-on-ubuntu-20-04 to fire-up postfix.

And this link easydmarc.com/blog/how-to-configure-dkim-opendkim-with-postfix/ .

I'll be dipped. It works brilliantly.

So I am ready to use the postfix-opendkim combo to do what gwia ought to do natively, which is dkim sign outbound messages.

Where I am stuck is on the precise steps on how to make the gwia use the configuration I just created.I know there is a route.cfg file, and some settings in the interface of gwadmin, just no clue how to set things up.

Much appreciated if someone could direct me on how to do it. I know someone will suggest using secure messaging gateway, but that is huge overkill for me as we have hardware firewalls scanning for virus and spam.

All, and I mean all, we want to do is dkim sign outbound messages.