According to Gmail's Twitter account, users who want to join the test can fill out this form. https://services.google.com/fb/forms/gmail-ios-beta/ The requirement to participate in this trial is
Google seems to support Outlook, Yahoo and some other IMAP accounts, but because the feature is still in beta, there may be changes.
There is no indication of when the non-Google account support feature will be released to the public. Based on Google's habits, it will be somewhere from a few weeks to several years.