Tracking the IP and Physical address of the machine originating an email message

Possibly, the Received headers show the client's IP address. Received headers are added at the top, so the topmost is the newest. The server is on a public network, so it can only log the client's public address. However, most ISPs don't include client IP addresses into the Received header any more for privacy reasons. You could also ask Google for details from their log but I'm not sure they'd deliver.

MAC addresses are never included in any headers, so that won't work.

If the mail originated from your network and you're properly logging connection details then there might be a chance to pinpoint the source. Extract the exact ingress time stamp from the according Received header (the lowest=oldest one mentioning a gmail server). With that time stamp, check your firewall logs for a user accessing gmail via SMTP. If users do so using HTTPS you'd require detailed logs, requiring SSL inspection.

If you lack any of the above there's no way to find the source, I'm afraid.