You install an update to macOS Mojave and, after restarting, you cannot log into Gmail using Mac Mail.
Create a new user and move your data to the appropriate folders.
This problem was first reported back on March 26, 2019, after the release of Mojave 14.4. We used the unfortunate event to discuss how you can use Update Schedules in your law firm to prevent problems like this from causing revenue loss. Since that insight was written, Mojave 14.5 has been released and nearly two months have past – and there has been no word on an automated fix.