The new Oauth2 support for gmail addresses only works for when your email address ends with “gmail.com”.
We also need this support for users of Google suite apps where you do have Gmail, just with your own domain name.
So at the moment, all these users need go through the “unsecure apps” thing with google. And when the admin of the google suite has turned that option off, all those people are our of luck and cannot use delta chat ;-(