If a Google account is not synchronizing items at all, you can look on the Nylas dashboard, under the Mailsync logs for the specific account if the following error appears:
Mail service not enabled
This happens when the account has the Gmail service turned off. This typically means that there is a Google account for this address, but not a mailbox. It is possible that the G Suite license is not enabled or paid for anymore.
The user can ask their Google Workspace administrator to turn on the Gmail service for their account. If the email administrator has switched the email provider, the user will need to re-authenticate with the new email provider.