Why is the Gmail SMTP relay server ignoring DKIM signing when forwarding e-mails from my VPS?

Question

As you can see in the attached screenshot, the e-mail sent from my Devuan/Exim4 VPS shows no sign of my server's DKIM signature. If you were to scroll down the screenshot, you will see Google's X-Google-DKIM-Signature, but the one from my own server. It has apparently been removed by Google. This is resulting in a DMARC failure, which is forcing me to set my DMARC policy to "none" so that e-mails will not be blocked. I can verify that e-mails sent directly from my server pass all SPF,DKIM, and DMARC authentication checks. I don't like the idea of having to disable DMARC checks to use Gmail's SMTP relays, which I need to prevent e-mails from going to spam folders on client inboxes. With a DMARC policy set to "none", I am running a much higher risk of of somebody forging my domain in a From address. This is my first question on the board. Thank you in advance for your constructive input.

EDIT: I might be getting closer to the answer here. The second screenshot shows the Gmail "original message" output as passing the SPF record check. HOWEVER in this case, I have already disabled my SPF record, and dkimvalidator.com as well as mxtoolbox.com have both verified there is no SPF configured. Gmail appears to be marking the e-mail as having passed an SPF check regardless of whether my record is configured. Also note in this case I have reconfigured my Exim4 server to act as a direct sender, as opposed to using a Gmail relay. The SPF check still passes in this instance, even with the IP of my VPS... this is just adding to the confusion, since you would expect the SPF check to fail with my server IP, since I am also managing the DNS in this case.