I thought gmail doesn't disclose sender's IP address? Or I was wrong? This is not good for privacy. Also, for fingerprinting you can obtain a GPU model via WebGL (helps in detecting a VM), and probably can scan for known browser extensions by trying to fetch extension-specific URLs. Some sites also scan ports on the localhost by trying to connect to them to find out which software is run.
The IP address is the smtp server of course, not the individual user.