Google's mail servers have been compromised for several weeks now. It's commonly being used for infected crypto spam (all those "new traderbot" emails with the attached infected PDF, for example). I'm not yet sure if these are just compromised GMail accounts, or if the mail servers themselves have been compromised. There seem to be some reports on AbuseIPDB of intrusion attempts coming directly from Google's mail servers.
I've tried reporting it to Google (eg via SpamCop), and Google declines to receive reports. I have been reporting it through AbuseIPDB as well. Here is one Google mail server that has had over 300 abuse reports:
https://www.abuseipdb.com/check/209.85.167.48
There are many more, and I linked to a few more when I posted about it here on HN over a month ago: