In order to successfully deploy the RingCentral Meetings Outlook Plugin via a GPO, you must
select the Basic option in the "Installation user interface options" of the Deployment tab in the Advanced Deployment option. When you add a software package to a GPO, you're given 3 options for the method of deployment, Published, Assigned or Advanced. Select Advanced and you'll be able to set this option. If you don't set to Basic, your users will get an error when trying to install that the installation is not safe and that your administrator needs to set the Basic user interface option.