As of May 30th, 2022, Google will no longer support the use of third-party apps or devices which sign in only using your username and password. Using an App Password will be a supported work around to allow you to continue using your Gmail account with REV23 Desktop.
Follow these steps to create an App Password.
When creating an App Password select 'Other' as the app type in REV23 as the name.
Once you have an App Password, change this in REV23 Desktop by going to Configuration > Virtual Receptionist Options and updating the password there, then sending a test message to verify that things are working as expected.
See this Google Support page for more information: