@rjkmadison,
I experienced the same problem, not with Gmail, which I don't use, but with Google Calendar. The problem occurred when or after I upgraded from macOS 10.13.6 to 10.14.6 over this past weekend. Everything was running fine in 10.13.6, Calendar was happily accessing my Google Calendar without clamoring for authorization ... and it was running Safari 13, albeit probably a version for High Sierra. After the upgrade of macOS I distinctly remember having to approve the update of Safari. I didn't notice until a day after updating Safari that I had a problem with Calendar accessing the Google calendar. I don't know whether that was the case immediately after 10.14.6 was installed, or whether the Safari update caused that. If the upgrade to 10.14.6 triggered the request to complete authentication, then I should have paid more attention and attempted to do so immediately. I guess I was just too focused on completing the upgrade, incl. any software updates, before deciding what the upgrade might have broken.