Are you using 2-step Authentication? With the latest version of gMusic you will need to generate an Application Specific password for more information please please read here: Application Specific Passwords.
Delete the app then reinstal it. That will fix it! Sorry for the inconvenience.
Google recently changed playlists a little. I am verry sorry there is not a workaround for this yet. I have already submitted gMusic 2.3 to fix this.
Email me! email@example.com