Recently I started getting errors from Calendar.app about my synced Gmail calendars. I haven't made any changes to my Gmail security settings.
Usually if Google needs to be authenticated for macOS apps it pops up a little portal window asking me to sign in. I haven't been able to trigger this window to popup.
It seems that at least one of my calendars is still syncing between Calendar.app and Google Calendar in both directions. All calendars are on the same account.
Just in case this is relevant, I recently (2-ish weeks ago) upgraded a different mac on my apple id to Mojave.
Update: I have cleared the iCal cache as per google support advice, as well as revoked authorization from google and re-authorized as per this recent thread on google support forum. The issue still remains.
The server responded: “403” to operation CalDAVAccountRefreshQueueableOperation.
Google Support thread for this issue.
Apple Discussion thread for this issue.